As far as the factory cd changer goes, I wouldnt spend a dime on it. Its old technology and you can spend about half of what you would to repair that one (you'd still have an old cd changer), as you would to get a nice CD deck to go in the dash.
Yes the key to a good sound system is to start with a good source. You could have thousands of dollars tied up in amplifiers, subs, mids, EQs, x-overs, etc. and power it all off of a $29.99 shaft-style cassette player?? I dont think so. Go and listen to a few good decks in the $200-300 range, you wont be disappointed.
Good luck, hope that helps...