For stereo, I picked up a Pioneer AVIC-F700BT for my STI (will fit your WRX just fine) for $350 after eBay's Live cashback. You can get the backup cam for that unit for under $120 after cashback, so you'd be quite happy with that. Check out the thread I JUST made on the unit (sans camera) here:
Woohoo! First 08 STI mod installed!