i'm painting my 86 Gt, and i goto Space age auto paint in Mesa AZ. Anyway the guy who's been painting for years there sold me a gallon and a half of paint and a gallon of primer. I also had to buy flexible primer and paint to do my front and rear bumpers since there rubber. So now i have a gallon of primer, gallon and a half of paint and clear. Also if you have any small dents or dings I use RAGE evercoat filler this stuff is awesome and i've never had a problem with it. I'm just waiting now till spring since its a little too cold to shoot a car.
It depends whether or not your doing the door jambs, engine bay, trunk area, etc. A basic job, a gallon and a half should do, but definately buy more paint than you need, because if you run out in the middle of the paint job you'll be in trouble. Also you can use the extra for touch ups down the line, also you don't to worry about the paint store mixing the paint alittle different when you go back for more. Good luck with you paint job.
dont use single stage paint unless you have to. i hate it. doesnt shine as good . but if you are staying same color and useing single stage you will need at least 3 quarts. if you use 2 stage[base/clear] i would say 2 -3 quarts and reduce it 50/50 you should be fine. and about a gallon of clear. all this does change if you have to paint door jams and engine bay.