I base my build off descent resists and massive amounts of Health regen per 5.
This allows u to stay in team fights almost indefinitely while keeping on Burning Agony permanently with out losing Health.

Early game i mostly just use Infected Cleaver to poke other Champs and to help with my farm. Infected cleaver works great as an early game poke with is good amount of base damage and its percentage health damage if other Champs have enough Health. And don't forget even though your playing Dr. Mundo you are still fairly squishy early game.

By now u should most definitely have your Spirit Visage, Boots, & part of your first Warmog's Armour, so u should be much beefier. This is a really good time for u to get some farm in and get some gold to help support the items you need for this build. In this phase don't forget that you are a good meat shield for your laning partner so if someone tries to gank you step in front and use your cleaver to slow and make them think about what there doing in your lane.

This is where Mundo really shines your use your massive Meat Shield self to keep your team safe and the enemy dead, & if any stragglers are left behind you use your massive speed to chase them down and end them, don't be afraid to use your Ulti just to chase enemies down, cuz don't forget you are Mundo the Undying.

I always start out with a Ruby Crystal because it build right into a Spirit Visage, and i dont like wasting gold on items I'm going to just sell later so Ruby Crystal it is. I usually get my full boots before i finish my Spirit Visage, I just like to run around minions and throw a Cleaver in the face of the other team. After i have both my Boots and Spirit Visage i like to go right into a Warmog's simply because at the levels you get it your health alone is good enough to deal with most the damage the enemy team is putting out. After the Warmog's its more about how the games going if they have an AD carry the is just killing everyone including u the definitely go for the Randuin's Omen, but if they have a lot of AP the go for the Force of Nature. Then for the last Item its always kinda a fun item i usually just go another Warmog's just so u can laugh at how much Health you have, but i have also went Atma's Impaler, Wit's End, or a Phantom Dancer

Pros
Your Ulti
It allows u to heal for a ton and stay in teams fight
Large Health Pool
With lots of health it makes it that much harder for a team to kill you when your chump blocking for your team

Cons
Ignite
50% less healing always hurts when you have your Ulti going
Lake of Taunt/Knock-up
Cleaver is nice for the slow but it still doesnt prevent the enemy from attacking your team mates.