Hello out there!
This is my build for Mundo, and i promise - as Mundo says himself, that you WILL be able to go as you please!! :D

I use CD runes in Glyph and Quintessence, while HP runes in Seal, and at last armor/mres runes in Mark.

The cooldown reduction will prove very useful with your cleaver, and your ulti.
Health is the best as a Mundo! - Theese runes is important at early game.
Armor & Magic Resistance - Well its unfortunate that theese are needed to be planted at Mark, but in my opinion, health in seal will do up for the low stats on Mark.

Well,, Ofcourse haste and ignite is chosen as theese are your summoner spells.
I perfer going all out on Offensive, with a single point in defensive for the poor but useful magic res.
This way you will deal a good amount of damage, and most of mundo's damage is magical which means that the spell pen and Havoc makes a positive difference :)

Well, there are a few considerations but not many. Some would perfer to skip dorans shield, and go for spirit visage at start, and sometimes i do too.
Depending on your money, and which opponents you are facing (AD or AP), you could choose to pick warmogs before sunfire, which i also do sometimes.
Finally your last item is a thornmail. Although if you aint facing many AD champs or if they aint very fed, i would recommend another warmogs.

If you happen to be hitting a lot at the enemies towers, you should consider picking your Masochism, but only if you happen to gain a lvl when about to attack tower or knowing that you will soon do.

Ghost FTW!!! you will always be able to escape, and also chase.
Combined with your ulti, Sadism, no one will catch you before 3-5 v You at endgame.
Ignite is awesome for FB, early kills, and also for reducing their healing abilities.

If your opponents have their brains with 'em, they are NOT gonna target you, but your teammates first, which means that you should never chase for long in a team fight, but tank as much as you can.
When ganking or chasing when its not huge teamfights, your cleaver will do the job hunting the enemy at a rate that your team mates can follow up to.

Cleaver for last hits and Burning Agony for pwning all creeps around ya.. - now you have a lot of cash
Care about burning agony though, use it depending on your HP left...

There isn't very much to say anymore :P, comment on the build if you want to, at least try it out if you are looking for an awesome build! :)

MUNDO goes as he pleases!!!