If you really want a decent, long lasting finish, I would highly recommend taking the body down to steel. My car had at least 4 layers of paint on it. It has taken me a very very long time, but I took it to steel with a DA sander and found several spots of rust starting. Very small spots (size of say....this , and areas of linear lines of rust that kinda look like a scratch). If I didn't get rid of these, they would have effected the final finish eventually. The roof was the most difficult/time consuming part.
I have my car in primer now and it's starting to look like a car again!!
You'll find the effort up front, worth it in the long run.
I'm sure RacerX will agree that prep work is 95% of a good paint job. The color spray is just a way to show off your prep work!!!
BTW, if your bodywork isn't perfect, I'd suggest painting the car white. It's amazing how much white will hide lazy bodywork.
Tim