looks like you've already made a decision, but i'd say definitely keep the E30. for one, they're bullet proof due to the extensive road testing that bmw put them through - unlike the E36, which was pretty much exclusively tested through computer simulations. also, the market is about ready to make E30s a real collectible. you'd regret it if you got rid of one now.
as far as sites for parts go, i like bavauto.com. i've also heard good things about pelicanparts.com, but i haven't had much experience with them.