[panhandler62]

For that you can easily add a decent head unit, amp and 5 speakers.
I would go with an 8" sub, preferable mounted in the cargo area, component speakers in the doors, an amp with something in the general range of 4 x 40 rms + 1 x 150 rms and a head unit you like the looks of.

Generalizing cost:
head unit $150
amplifier $250
sub + enclosure $150
speakers $300
cables, hookups -n- sundry $100