I main rengar, and can answer most of your questions. Firstly, Tankgar is acceptable, but not that strong. There's just not real reason to build him tank. He has very little utility aside from straight up murder. No peel, no hard cc aside from his conditional root.
Rengar is a good pick against (I feel) anyone, really even though I don't know. His root is really solid if you can land it, but otherwise he has a tough time against champs with escapes if his ult burst can kill them, although it normally can.
With rengar, I focus on farming until 6 and banking whenever ult is up. Also, if an enemy is overextending get five ferocity and lead off with bola root (this can be done pre and post 6). Nice, free kill 9 times out of 10. For build, I go rangers/stalkers w/warrior, Ionian Boots of Lucidity, then rush youmuus, then RH, then LW, then I tend to round off with a spirit visage. Cleaver/Maw are also good on him.
That's all, folks. I hope I was able to help!