There's much more to paint prep than most people think. It's 80% of how the final job will look. First, I'd suggest you let a body shop do the prep work. They're more experienced, have the right equipment, and know what to look for. If you're serious about how your car is going to look, this isn't the time to save a few bucks.
That being said, much depends on the condition of the original paint/body. If it's pretty bad, start with a #220 sanding, followed by a #320 sanding. You'll need a D/A sander for most of the car, the smaller areas require hand sanding. Do not try to remove the original paint any more than necessary to get it smooth. It's the best base you could have. Next is too fix any small dings/dents. These small areas will have to be ground down to the metal, and the metal scuffed with a course paper like #180.
After sanding/shaping the plastic with #180,#220, then #320, these areas will have to be primed, as well as any areas showing bare metal. There's no sense in priming the whole car. Sand the primer with #320. The painter will still have to give it a final sanding just to get it clean before entering the spray booth, then use a Tac cloth once inside.
This is absolute basic info. Every car I've painted was judged on it's own needs. But, they should all be the same when entering the booth. The trick is too know how to get each car to that point. For example, lacquer paint sinks really bad, but it's nice to work with and shines like hell. But, a final sanding of #400, then a wet #400 sanding was always necessary because of the sinking. Most paint jobs today are enamal, so that isn't a concern.
As mentioned earlier, and I can't stress it enough, this isn't the time to save money. Let the pro's do the prep work.