I'd say painting the car first would be a bad idea if you're going to do anything else (other than aesthetics) to it.
Paint it first, and you're going to be really pissed when you're installing your new lowering springs or that new exhaust and one of your idiot friends clumsily puts one of his greasy paws on the paint or fumbles a wrench and puts a big ding in it. In other words painting the car last would probably be a better idea. Plus, if you decide to put a body kit on it, you'd want to have it all matched (I assume), and why not do all of that at the same time?
:monkeypis Mustangs
:monkeypis Stupid No-Driving Old People in Beamers!