How do I build AP rengar?

You don't

How to build ap Rengar: don't do it. Go AD and win.

If you're just SET on it (and, for real, don't do it), you'll want the AP jungle item (pick Red Smite), followed by a Lich Bane. If, by some miracle, you're still getting gold, get a Gunblade. There's basically no way you're going to get past that item, but if you do, Nasher's and Rabadons are the way to go