You'll have to decide what you want to do WITH the truck before you really decide what to do TO the truck.
Starters are bearings and electronic speed control (esc). Next is usually oil shocks, and then some sort of chassis. There are approximately one gajillion chassis for the clod now, depending on what you want to do with it, or you can make your own.
Better motors are also fun, but that may be better saved until the truck is set up to handle them.
Brian