Sanders come in all shapes and sizes. On most large repairs, you will be spending a lot of time with these tools, so lets talk about them.
For large, relatively flat surfaces, an orbital or dual action (DA) tool will work well for grits below 400. Above a 400 grit, plan on using a DA.
For surfaces with compound curves, often the orbital won't cut it, as it only works on part of the surface. A DA fitted with a high-relief foam pad can do pretty well in these situations.
On intricate detailed parts of the hull, plan on doing some hand sanding with a block.
I am a big fan of air operated DA's. These little puppies do a GREAT job. They are both speedy and have a nice "touch". The last thing that you want to do is cut through an otherwise perfect gel-coat finish! Quality pneumatic DA's can be found for around $60. The pads for these tools can be either Velcro or adhesive. It depends on your preference. I usually use adhesive for the low grits and Velcro for the high grits.
Expect a pneumatic DA to require around 5 SCFM at 90 psi. Don't forget to oil all of your pneumatic tools per the manufacturer's recommendations.
A DA that I really like is from Ingersoll Rand. The 311A DA sander does a fantastic job. A feature that I particullarly like is the adjustable regulator ON the sander itself. You can actually control the speed without having to adjust a separate regulator - saving a lot of time!
Several manufacturers also offer random orbital units. These are mostly aimed at woodworking, but do pretty well for most fiberglass work. They are not true DA's, and don't give as good of a finish, but if you are planning to paint (or hand sand your gel-coat) they will work. These are widely available for around $45.
Belt sanders are of limited use in fiberglass repair work. They are simply too fast, and it is EXTREMELY difficult to control where they are cutting. They are best used in the early, demo stage of a major repair. A word of caution here:
Be sure to notice the way that the sandpaper is to be attached to the sanding disk! Some sanders use "Hook and Loop" (Velcro) to attach the sandpaper to the disk, while others require glue-backed paper.
Both work very well, but you must be sure to get the correct sandpaper.