If you're going to paint the car the same color, the only place you need primer on is areas that are sanded through to metal or on areas you are going to be doing bodywork on.
Is that paint a factory paint? If so, you don't need a sealer, just a thorough sanding before painting. If it's an aftermarket paint already, and you don't know what kind of paint it is (urethane, laquer, catalyzed enamel, uncatalyzed enamel, synthetic enamel), you'll probably need a sealer over the repainted areas to keep the new paint from reacting.
Usually, however, on a factory paint or old respray, you can get away with using a DA or hand sanding with 320 grit to level the surface and get it uniformly dull, then hit it with the new paint. If during sanding you go through to the metal anywhere, you wil need to hit that with primer, then hand sand it to prep for paint. Since a lot of colors, especially metallics, are kind of transparent, you may need to hit primed areas (or areas that are lighter after sanding) with a couple extra coats of color first, before spraying overall.
Here's a thread that I described a bunch of prep work in. It was based around a question about Maaco, but a lot of it applies here, as well: