Some 5x8" coaxials (speakers with mids and tweeters built into one) will drop right into your doors. If you want even better sound (but also costs more), you should consider a 6" component system (which includes seperate mids, seperate tweeters, and seperate crossovers) all for one price. This will also require adapter plates to be fabricated for your factory speaker locations in order for the 6" mids to fit. Once you pick out the speakers you want, then you can start looking for an amp that supplies them with their recommended RMS power.

Brands you might consider, in ascending price range:

Alpine
Infinity
Polk MOMO
Diamond
JL
CDT
Boston
Focal
.....just to name a few.