I used 120 grit sanding discs on my hand orbital sander to start. Any orbital sander will do, just make sure to buy the right sandpaper discs for your sander. Mine was a cheap Harbor Freight/Northern Tool brand that I inherited with the house... was left in the garage. 3M sanding sponges work great for inside curves and tight areas. The Pro grade ones last longer. Medium is about 100 grit, fine grit ones are closer to 200, if I recall correctly.
100-120 grit will take off any deposits or help prep an area for patching with bondo/plumbers epoxy. If you need to patch any holes or thin areas, sand around them with 100-120 grit, rinse well with water, dry then patch and resand with 100-120 to level the area. Even if you don't have to patch at all, go over the whole thing with ~120 grit first. Then go over the whole thing, anywhere you're going to paint, with 220 grit. Then wash with dish soap, rinse THOROUGHLY with water. Then you can move onto first coat of paint.
This has some good info on surface prep. The better the prep, the better the finish.