It also depends if you want to keep the trunk/T-top storage. If not, I'm pretty sure you can get 2 15's. If you need to keep the space (like me) There's the little side pocket, driver side opposite the spare. If you have the Monsoon system, put some 8's in the sail panels, and some good full range in the from doors, get 6x9's in the very back and put a badass 12 in the little side cubby. If not, You can get 2 12's or 3 10's in the space between the back seat and the place where it drops down into the trunk. Throw at least 1k watts on them and call it good. Personally, I'm getting Focal components up front, tweets up high in the doors, 6's in the bottom, (probably) Kicker Comp 8's in the Sails and some high end 6x9's in the rear panels, and a RF 12 in the rear cubby. But I'm buying all new so it's going to cost about $2k... If you're buying online just make sure you don't get bent over.
Doc