I've tried this build out for a while, both in normal games and practice just for fun. It's can be pretty ridiculous if played correctly. Initially it was something I started doing as a joke but as I saw the viability in building Mundo this way I started to play around with it a bit more. It's pretty unique.
As far as runes go, I have an entire book dedicated to crit... at level 23 I'm at 12% crit from runes alone.
Skill wise, I always go for 'Q' first and then 'E', I max out both as soon as possible (getting ult when possible) and then start with 'W' at level 13.
My itemization is pretty random, I tend to go with what feels right but as a base, I usually start building an avarice blade and two health pots. Usually by level 6 I have 2-3 avarice blades (this might sound like a joke lol but it works), but it really depends on how early I draw first blood. If you'll notice from the video, I tend to skip boots for the most part due to the fact that I tend to be pretty good with cleavers and I usually pick up exhaust.
An average game's items would look like such:
Brawlers Gloves and two health pots
2-3 avarice blades ( I usually don't go back until level 6ish, at this point I have 2-3 kills laning with a partner and 1-2 soloing)
BF Sword -> Infinity Edge (crit damage helps IMMENSELY)
You can usually start selling off avarice blades mid to late game and replacing with bigger crit items like Phantom Dancer or the Impaler, if you find that you're getting focus fired you can always pick up Warmogs, Frozen Mallet or any other +health item you feel will be right for the situation, otherwise I continue to get items high in +dmg.
I tend to play VERY aggressively as Mundo, which usually helps me out (or screws me over if I'm countered properly :P).
I know this guide may be a tad vague and short as I wrote it up at 4am whilst slightly intoxicated but if you have any questions/concerns/feedback feel free to reply or add me in game.
This is a fairly short video showing how viable he is as a nuker, the quality is **** due to the fact that fraps tends to generate 2 minute clips bigger than 4 gigs in size so I wasn't really up to waiting for something to reduce it in size and went the easy route.
http://www.youtube.com/watch?v=zoHLxI80VF4
Let me know what you guys think.