Which champion do i buy if i play jungle?

Jarvin. He can be built from full tank to full ad, is first pickable, and works in a number of team comps.

Ideally you'll want a tank jungler, an assassin, a bruiser, a hard carry, someone with wave clear or split push potential, and an AP damage jungler.

Zac, Vi, and Warwick are all insanely fun to play

Khazix and shaco with new "assassin" runes + duskblade = gg

If you want fun: If you want elo:

Buy the 450 IP, eh sry 450 BE, jungler champs as you won't get enough blue essence now anyway to afford anything more expensive.