Well, I've been playing Dr. Mundo for awhile, and I can say one thing: Mundo could work pretty well as a hybrid. You might ask for reasons, and I have reasoning right here:

1.Mundo doesn't scale well with AP at all, the only skill which remotely scales with AP is Burning Agony, and to be frank, not too many opponents will stay too close to an AP Dr. Mundo if he has Rabadon's Deathcap and heck knows which other AP items.
2.Mundo has an inbuilt Force of Nature effect on himself! The more HP Dr. Mundo has, the more he regenerates after every second!
3. Infected Bonesaw's damage scales with the amount of HP the opponent has, and doesn't scale with AP whatsoever, but does indeed work with Spell Vamp quite nicely!


And this is for short ladies and gents.

Oh, and this is my first guide, constructive criticism would help :3

Now pros and cons... well this will be tough
Pros
Mundo will regenerate pretty fast after getting Warmog's Armorand Force of Nature.
Some more hit points than normal magic Dr. Mundo.
Health regenerated after every Cleaver which hits.
Late game monster.
Cons
Hybrid means not too much damage from spells (I mean spell, because Heart Zapper is the only AP scaling trick Mundo has) or having too many hit points
Might be troublesome to start with at first.
Getting all the items means entering massive killing sprees and assists.
Early game wuss.

Now, here I will be explaining the items, what and why!

Sorcerer's Shoes
Self explaining really, the Infected Cleaver hits hard, and most opponents will get some items to stop at least some of the damage, Sorcerer's Shoes will do that trick of reducing their magic defense with Archaic Knowledge and the runes even better!

Hextech Revolver and Hextech Gunblade
Pretty self-explanatory, it has Spell Vamp and can be upgraded later in the Hextech Gunblade if necessary for even more item potency!

Warmog's Armor
Pretty self-explanatory, more hit points, better effect from the passive, and further increased by the Force of Nature. I dont' find any reason not to include this guy in any Mundo build.

Force of Nature
Further increase of Dr. Mundo's passive, this will yield very good results in survivability!


Randuin's Omen
I replaced Spirit Visage with this one, because I've noticed a major fault. Melee classes will cut through Dr. Mundo's hit points like a hot knife through butter. And so I've added this hefty item!

Atma's Impaler
Now, this item is good especially for the Critical Strike and the 2% health turned into damage buff. What this means? The more hit points you have, the more damage Atma's will give you; Atma's will beef up your melee damage considerably, and think of it as an upgrade to Heart Zapper, because you are in melee range and Cleaver will be mostly on cooldown, you will have a better whacking stick power! Aaand the critical chance is also nice too. Also note that when you get this item, you will usually be level 18, and have Blunt Force Trauma up. Activate the skill, get at around half health, activate Maximum Dosage and /dance over your opponent's sorry corpses.

Purchase Reordering
Warmog's Armor-> Atma's Impaler-> Randuin's Omen
Now; there is at least one thing that you should keep in mind. If you bought Warmog's Armor and think that you could use some more damage, you could get Randuin's Omen later and get a fast Atma's Impaler/ Why you might ask? The synergy between the two items is marvelous, Warmog's give what a healthy growing Dr. Mundo needs, and Atma gives critical chance, some armor, and damage scaling with your hit points. Only get this if you think that you'll survive without the extra armor from Randuin's for a while.

Boots
Sorcerer's Shoes
These guys are the basic boots if you're doing OK in a match.

Mercury's Treads
If the other team is CC heavy, and your Heart Zapper doesn't seem to keep up with all that abuse, I suppose it's time to get these guys. They also provide some magic resist if you're having trouble with caster characters that can stun you off.

Berserker's Greaves
To be honest, I rarely use them unless I'm going another hero that could benefit better from this item. It raises your attack speed, but this thing shines only in later game after Blunt Force Trauma is maxed out.

Boots of Swiftness
These guys are awesome with Maximum Dosage. And if your enemies get away too often, why not get a pair and use Infected Bonesaw at a safe distance while they're slowed down?

Ninja Tabi
Now these little guys help you dodge melee attacks, if you have problems with guys like Master Yi or Tryndamere, getting this item would be a good idea. Along with the Thornmail you will be a little harder to kill, and have more chances or running away.

Other items
Thornmail
I usually get this when I'm against strong auto-attackers, like Master Yi, and honestly, it helps a lot especially if he gets tons of attack speed items!

I don't have too much to say here.
The Greater Seals of Evasion will help triggering the Nimbleness ability, and 10% movement speed is always welcome, especially in getaways!
The rest of the seals are magic oriented with cooldown and magic penetration, for the Cleaver to dig in deeper into your foe. Also, earlier Maximum Dosage! Who doesn't love that? Your opponents.

When the more knowledgeable people will see my 9/9/12 mastery build, they'll think I'm nuts, and -1 this faster than you would say "What?"
For those who didn't -1 and read on, I will be explaining why and what.

Offense
Deadliness
Now, this might not be too much, but 2% combined with Atma's Impaler 18% will make a total of 20%; that means a hit in five will crit!

Cripple
Self-explanatory, it is a two-way enhance of the Exhaust, two way being either tailing or running from opponents; this thing saved my *** so many times it isn't funny, either just fire and forget style, or fire and cleaver-ate (...I failed) your opponent.

Sorcery
A reduction of 3% in cooldown is always welcome, believe me.

Archaic Knowledge
This thing, combined with your Sorcerer's Boots, will turn your Cleaver into a serious threat for most people!

Defense
Resistance and Perseveration
Prerequisites for the bigger stuff, nuff said.

Evasion
Dodge more, and with nimbleness will also help you either survive or murder!

Nimbleness
This is the 10% movement speed after every dodge trick; now this will either save your purple buttocks, or demolish the ranged characters who have a bad habit of shooting and running!

Utility
Good Hands
Revive a bit faster, always welcome!

Perseverence
This thing increases health regeneration, so in my point of view, it works a charm with the other items listed up there!(there being ^)

Haste
Longer ghost, and more speed included, what's not to take about this one?

Awareness
Faster leveling, better cleaver, faster Maximum Dosage, faster you! This will also help that most people don't like Dr. Mundo.

Utility Mastery
This will increase the buffs you can receive from the jungle by 30%. Imagine it like this, after three buffs of one kind, you will actually have four buffs worth of time (minus some seconds)

There isn't much to say about skill sequence. Infected Bonesaw is your bread and butter early game. This can also be used to scout the top and bottom lane bushes, lining the cleaver so it can hit most of the bush, you can throw it in; and if you hear that it hit something, prepare to engage! Be sure to train using this trick, so you don't waste more than 2-3 seconds doing this scouting trick, waiting too much means opponents seeing you and run after you.

After you get Heart Zapper, be sure to have it on when you either go between some minions, or you are going to get ganked; with some hope, you will survive due to the effect of reducing cc (crowd control: stun, blind, disable, etc) statuses. Be wary that you should always keep an eye on the health, and another on the minimap (works best if you're a chameleon, but if you aren't, you can throw a look on both, if you get what I mean); if you abuse that skill too much, it will eat most of your HP whole, and make you an easier target. If you don't activate it at the right time, you will probably get ganked so much with CC effects, you will most probably die.

Blunt Force Trauma is not too useful, because making it very useful means lowering your hit points at very low amount; and that is not ideal by any mean. But when you have to crack Masochism, think about this: will they lower your life low enough to make it work a charm? And if it does, are there any team mates nearby to who you can run so you can save your life if you get too low? If the answer is yes in both questions, you can use it without a fear... I mean without you having a fear, because your opponents will mostly be in a pickle after you leave! And I also use this only in endgame, because only them I am durable enough to survive using this, and combined with the At-Mog's combo, it's pretty freaky.

Maximum Dosage, the true face of Dr. Mundo. This thing will steal 20% of your current life (imagine 20 hp taken for every 100 hp left you have), but it can heal up to 70% of his whole life! And as it wasn't enough, you will run up to 35% faster. A homicidal maniac is you! Just be wary when using this, too less HP and you will get killed, and if you use it too early, opponents will avoid you if they know what they're doing.

My skill choices are the following:
Ghost
This pretty skill will make you run like a rabbit hopped on espresso! And did you ever see a Ghost affected Dr. Mundo while he is on a Maximum Dosage spree? Probably not, because you would be dead by the time you wake up and ask what happened.

Exhaust
This can be used in two ways:
-Stop a running opponent from getting behind a turret and finish him off
-Stop a running opponent from finishing you and help you land an extra cleaver before you run away like a mighty deer!

Anyway, there are a few other skills that I'm also going to rate; if you're not going to use Exhaust or Ghost, don't forget to put the point from Cripple or Haste elsewhere.

Heal
Useful in the early game, but after... you will probably use it to heal minions or weaker team mates.

Revive
Not so useful really, you aren't supposed to die.

Smite
Use only if you're going to try go jungling, or want to have an anti pet spell for other heroes like Annie's Summon: Tibbers.

Teleport
In case you want to move faster across the map, this is your ticket! But you have Maximum Dosage to help, and it will also heal you.

Cleanse
You have Heart Zapper, so I'd say nay.

Fortify
If you like hugging your turrets like you like hugging your life, you could take this. It helps a turret being demolished too fast.

Clarity
Dr. Mundo doesn't use any mana, so you should totally avoid this one.

Ignite
To be honest, this one is only good if you have the bad habit of letting people disappear with low hitpoints. This also halves the healing from effects; and by effects I also mean Maximum Dosage.

Rally
Another one I say no to; it isn't that good to be honest, and another skill could easily replace this one.

Clairvoyance
If you decide that you don't have enough map vision, get this, it should help you. It can also help you keep track of some people who ran away from you, thus you can take a shortcut and cut his way to his base!

Flash
Well, this skill is pretty helpful in either attack or defense, and you can also hop over terrain with this skill. But there is also the rumor that this skill will be removed at a later date; but until then, if it will help, use it!

Dr. Mundo shouldn't be always hung up on either HP+HP regen, or just AP with Magic Vamp and Magic Penetration, he should be a combination of both, maximizing the potential of this class.

I hope this helped you, and this is my first guide. I will update it if anything happens to Dr. Mundo, and I will also add even more by how much time I manage to get.