I replaced the factory stereo on my '08 almost four years ago, and replaced the rear 6x9's with Kickers. Total cost about $200, no kidding. Don't let them tell you you have to spend a fortune. This assumes you aren't gonna put in subwoofers and separate power amps etc.
The stock factory radio is a prehistoric POS. Even a $55 Dual head unit from Amazon has USB and SD slots, as well as cd player, stereo plug input and twice the power output. Go to Crutchfield to see what adapters you need: wire harness, door chime module, ISO slot adapter etc. Then buy em cheap wherever, just get the right part numbers. Replacing the rear speakers is easy with the right tools: see Youtube video about getting the back seat out.