i suggest go with a good set of separates for the front speakers (jbl, focals, dls, infinity) then its up to you if you want two subs or not. but i'd go for 12" subs. one would do and its enough to give you a good deep bass
and put the sub at the trunk

a basic "abc" box would be good for starters
given that set up, a 4 channel amp is enough. 2 channels for the the front separates and the remaining two will be bridged for the single 12" sub
if you want 2 subs, a 6 channel amp would be needed. 2 for separates, and 2 bridged for 1 sub and the remaining 2 for the other sub
hope you follow
if you have extra cash, you can buy coaxials speakers for the rear to improve SQ further. it has better quality obviously over the stock ones - so there is still a big improvement
hope i got to help