Rengar player starting out, PLZ HELP

SO, uh, just bought rengar. I know his combos, basic map roaming, and jungle clears but a couple questions, id really appreciate people answering : ^)

1. is crit rengar actually better than just ad rengar?
2. is ap a legitimate build or just something to troll with
3. if you do crit, is there any way to get 100 crit without selling boots? (both jungle and lane)(also, don't say "Get rid of yomus" its too good to have on an assassin)
4. any last tips and tricks? TYYYYYYYYY when u see the little exclamation point GOTTA GO FAST