I do my Dr. Mundo a little different. I focus more on Health Regeneration through Spirit Visage,2 Warmog's Armor, and a Force of Nature. Along with either Ninja Tabi or Mercury's Treads. Also a Randuin's Omen for CDR and even better chases.

+ 1
several comments though
You need to get your Atma's Impaler earlier, because just relying on macochism and Infected Bonesaw alone isn't enough damage. YOu may say "But im a tank! I don't need damage!", however, Dr. Mundo is a kind of special tank because he has no taunt, or no stun, unlike champs like Amumu and Rammus.

So to create threat, you need damage. Also, another note. Boots of Swiftness are no needed for dr mundo because honestly, its just 20 more speed, whereas you could buy Mercury's Treads which are a whole lot useful.

Another thing, Thornmail is overrated, especially on dr mundo. Do not get it unless their team is full of fed AD carries.

If i helped, please check out my Janna build? Pwetty Pwease? :3