in this order.
-Headunit-look for equilizer bands, sub out puts, aux output, functions, etc
-Speakers-front need to have good mids and highs, infinity, MB quart, boston
rear need to have bass and maybe some highs but you wont really hear them
I would suggest components for the front.
-Subs, Amps, Wiring- different strokes for different folks i guess. search around. Mainly Sound qaulity and SPL are the two types of bass. w/e. search. vague question to answer.