I don't see why you need a 12" so long as whatever sub you buy reaches the lower frequencies. The key is building a properly tuned enclosure that maximizes the potential of your sub. When I do my installs, I like to maintain as much trunk space as possible. My focus install utilized the space where a rubbish factory sub sat (had a custom fiberglass box built) and a false floor where the amps sat. I had tight bass response and could still haul 10 cases of beer.

As for the rear, buy an inexpensive set of coaxs that upgrade the factory speakers.