Most people would like to build AD on Rengar and that's not quite a bad idea,but most of the times top and jungle have to be a bit tanky.So with these items he has damage and tankiness.

Of course on Rengar you should have more AD masteries than tank.

Runes are probably the usual you'd get for a fighter/adc.

Playing with Rengar top mostly depends on the opponent you get.If your opponent is tanky melee then just farm and attack when he comes close to your minions.You can also use bushes to help you escape fights or engage.

However if you're against a ranged champ things get tricky and skill will probably win the match.You need to stay near the minions to gain experience and then wait in front of tower to actually kill the minions otherwise the ranged champ will kill you.Do like that till lvl 6 or till you get a good gank from your jungler and from lvl 6 it should be easier and you can even help others.