Disclaimer: I'm not a luthier, and my use of belt and disc sanders has been mostly for metalworking.
Michelle, what do you mean by 'bench-type'? Generally, I don't like giving up bench space to a machine, especially one that's going to generate dust that'll cover everything around it. (Dust collection is good to have, but it's never 100%). If you have the shop space, I'd recommend a free- standing tool.
Resist the urge to buy a cheap offshore machine. They're often poorly balanced and consequently don't run true or smoothly. I have a decades- old Delta sander, 6 x 48 belt and 12 inch disc. These are common abrasive sizes, you'll find a wide variety of types and grits available. Also, don't skimp on power, a machine of this sort needs at least a one horsepower motor. (That's one North American horsepower- they must have awfully small horses in Taiwan...) A sander that slows down under load is a frustrating machine to use.
In this case, bigger is better. You can always sand a small part on a large machine.