A fifth wheel will require a pickup with plenty of payload capacity. If you see a couple of floor plans you like, find out the pin weight. Then look at pickups that can carry a fair bit more than that weight. You will probably be needing a 3/4 ton or 1 ton truck. Diesels are great for towing, but in used pickups avoid the 6.0L and 6.4L Ford diesels (the newest version, 6.7L, is okay) and personally I would also avoid the Ram hemi gas engines.
There are a few smaller, light weight fifth wheels out there that a half ton pickup could handle (Scamp comes to mind), but not many.