As performance wise everyone will say either get Cold air intake, header and exhaust. or they might tell you to save up money for a turbo kit. Also one of the first things to do is suspension. drop it about 1.5-2.0 inches it will handle alot better.
as for the amps and subs depends on what u want and what kind of music you listen to. if you want loud bass 2 12's possibly alpine type r's. if you want something less just to fill the missing sound then go with one 10". amp wise you have a slew of choices just match up the power ratings with the subs and you will be set