It all depends on how much money you're willing to spend, and what kind of sound you're wanting. Do you want it to be loud, do you want alot of bass, do you want detail? You can install a good sound system for under $1000 (new deck, new speakers, subwoofer(s) ), or you could easily spend 5k on an awesome setup.

As far as CD players go, I love my Kenwood eXcelon X817, I'd recommend it to anyone. Only drawback is that it's a little pricey for some (apx. $500).