This assumes you don't have bose or onstar, and if any of this is confusing I'd give Sonic Electronix or Crutchfield a call to make sure you get the right stuff for your head unit. I would bridge channels 1-4 into just your front speakers, leave the rear stock and powered by the hu for now.
The above is ~500. The next dollars I would spend would be for this
Subwoofer box is 10 inches high 12 inches deep so check for clearance