thank - will try to incorporate your advise

This guide appears pretty good, but if I were you I would go more in-depth on the situational items. It seems you have listed every viable item for Mundo... and a few other items.

Your core build certainly looks fantastic, and the skill order seems viable. Most people prefer his W over his E for jungling due to the constant AoE damage, but I could see this working better in the new jungle, now that they've buffed the main monsters and nerfed the small ones. Good idea.

AP is generally not built on Mundo because he has only one ability that scales on it, and it's only 20%. Perhaps this would result in slightly faster clears, but Mundo clears very quickly already. Magic penetration can be useful on Mundo, but it's better to leave it to runes, and MAYBE boots. Using an item slot for an early-game item that Mundo only benefits halfway from seems illogical to me. I don't quite see how you could afford to do it, especially when jungling and getting low amounts of gold.
Another thing is that jungle monsters don't have magic resistance, so magic pen would be worthless in the jungle, and only useful in ganking.

In my experience, it's much more productive to build Mundo with tankiness and/or attack damage than magic pen and ability power.
Not that that's impossible, of course, Fuggernought has an extremely viable guide on an AP Mundo.
However, I don't think it would work well for jungling.
Just my two cents, I'm not saying the guide should be downvoted because of it.

All the situational items listed that don't have AP on them are absolutely viable for a jungling Mundo. Maybe in some circumstances an abyssal scepter could be useful. Even a haunting guise could help out a bit early game if you had the extra money. Rylai's would only make your W slow by 15%, and I THINK make the slow on cleavers more severe. Not really optimized for Mundo, the way I see it.

EDIT: Just looked at the guide again, looks much better in my opinion. +1
However, I'm sure Mundo could use a bit more damage early on for ganking; these items essentially give nothing but tankiness until the Frozen Mallet, which is great, don't get me wrong, but maybe an earlier Phage would be useful for picking up a few kills early to midgame? As it is, you'd be relying perhaps a little bit too much on your friendly laner to help with the gank.