I am making this guide because I feel Mundo is one of the few champions stacking gold items on is a good choice. Now I know a lot of people don't like gold items, but since mundo just needs health early in the game, it doesn't slow him down at first. So please, try the build and if you don't like it then, feel free to tell me.

So lets stack some gold now!

Marks:
Right now I use , to make my Infected Bonesaw hit harder.
Though you could use whatever you want here (health, health per level).

Seals:
Greater Seal of Vitality, because you know, MUNDO goes where he pleases!

Glyphs:
, because we won't be building any magic resistance items early on, you will want some scaling resistance.

Quints:
Now this is the important part of this build.
Greater Quintessence of Averice Math will follow later.

Pretty straightforward.
Maximum Dosage every time you can, because you want to heal.
Infected Bonesaw next, to harrass and last hit.
One early point in Blunt Force Trauma will make your auto attacks hit harder and give you a nice damage to towers. The other points in this will come last.
Heart Zapper is a skill that is well suited for tanks. You will always be in the middle of the fights and damage everyone around you, plus the CC reduction is great. With our health regeneration, we can keep it on all the time.

Ghost will help you chase down anyone and escape early game if needed. Together with the speed boost from Force of Nature and Maximum Dosage you will be running with superspeed for about 30 seconds.
I take Ignite for some early game kills and to disturb their healer/lifesteal champions lategame. If enough people on your team have ignite, feel free to change it to something else you like. For example Exhaust (Mundo loves slow!).

Lets do some math here.
All the gold gain starts at 1:30 when minions spawn. So when I say 5 minutes, I mean 5 minutes after minions spawn. And 10 minutes is actually 11:30 and so on.

Passive gold gain is 7 per 5 seconds (14 per 10).
So in 5 minutes 7*12*5 = 420
And 10 minutes 7*12*10 = 840

Our 3 Greater Quintessence of Averice + mastery, gives us 4 gold per 10 seconds.
During a 20 minute match this is 4*6*20 = 480 gold (a free ).
After 35 minutes 840 gold.
And after 45 minutes 1080 gold.
This doesn't seem like much yet. But every little bit of this helps.

We start with a , so we got no money left. Next time you wanna port back is to get your first + with a total cost of 850 gold.
Say you are doing really bad at last hitting minions and you get only 10 in the first few minutes. At an avarage of 20 gold per minion, this will be 200 gold. And in 6 minutes you will have 18*6*6 = 648 gold. So in 6 minutes you can get your first back.

This will boost your gold incom from 18 per 10, to 23 per 10.
Now in your next back, or backs you want to buy 2 more and your . If you need multiple backs, the order is up to you.
But say you buy them all at once and really aren't last hitting much.
You got 20 more last hits, worth 400 gold and there isn't much fighting so you got 400 gold from kills/assists.
You need 2800 gold.
After another 15 minutes (+ the previous 6) that is 23*6*15 = 2070.

And this will change your income to 33 per 10, which is 198 gold per minute, just from doing nothing.
Now you might say, what if they have a caster heavy team? I can tell you this build still works then. Maybe buy your boots faster, but remember we also got the magic resist per level runes. And this is Mundo, he loaves to SOAK damage. Which is what all the bonus health is for.

And remember this scenario is a really BAD GAME! Most games you will have this core build in 10-15 minutes. If you can push some towers fast, even faster.

Now what is up next? More HP please. More health syncs great which his Maximum Dosage.
So we will get ourselves a Warmog's Armor, 3000 gold.
Just by standing around, no last hits and no kills and assists or enemy towers down. You need about 15 minutes to get it. 33*6*15 = 2970.

This is the point where you stop dying. Up to now, some deaths were possible, but now you just soak soak soak. Though there magic damage is getting pretty strong at this point so some magic resistance would be nice now.
So we will get a Force of Nature. Magic resistance, movement speed and more health regen the more health we got? Yes please.

Right now the only way you can get killed is by letting al 5 enemies hit you for 30 seconds and then you can still kill one or two of them with your Infected Bonesaw.

Next items are up to you. Some great choices are:
Randuin's Omen
Sunfire Aegis
even more Warmog's Armor (got to love 7000+ hp Mundo).

Now when the game has ended, look at the gold gain table. You should be in the top three of champions with the most gold. If no one is overly fed, you will probably even be number one.
Most games I leave (most of) my enemies behind me with 3000+ gold.
Which is a Warmog's Armor or any other fully expanded tank item.

So what items should you buy next? The current build is allready very costly, but unless they surrender in 25 minutes, you will have enough gold to buy more.

Of course which items you buy next depends on if they have lots of magic damage, or physical damage.
Most of the time you just want more health though.
Only if you really need more magic resistance, you could buy another forc of nature.
Otherwise I would suggest:
Sunfire Aegis Health, some armor en more AoE damage
Randuin's Omen Really great if they have a lot of auto attackers, because it will slow them down. And you don't have to sell a Heart of Gold for this.
Warmog's Armor Mundo really can't have enough health. If you have enough magic resistance and armor and you aren't dying, get a second one.
guardian's angel You really shouldn't be dying (a lot) right now, but it gives some magic resistance and armor and in the rare case you will die, you just revive.
Atma's Impaler If you are feeling funny and want some damage too. You should be able to have 5000 health which gives you 100 bonus damage. And some nice armor too.

What I think you shouldn't get:
Banshee's Veil Mundo really doesn't need this. You got CC reduction and you can soak damage all day long. Really, one spell block every thirty seconds is not going to help you.

I will update this guide with more information, playstyle, team work, tips and tricks. But all of that will be later.

Please try it out and don't criticise it if you haven't tested it. Mundo is one of the only champions this works on.