Most all of the parts are inexpensive, from the power supply to the memory, PC and needed parts, etc. should come in under $500 pretty easily. It's the LCD screen that will cost you. It needs to be VGA to be readable.
There are not many full blown kits that are ready to go, but there are some. Most though are underwpowered PC's or not good enough quality of parts, or just plain too expensive for what they are.
If you just want a hard drive based player, a laptop may be the easiest way to go. It is inexpensive and complete. All you would need is a power inverter to move your 12vdc to the 120 volts for laptop power, and an adapter to get the audio into your system (FM adapter or a tape adapter for example).
Also, a simple Ipod would work for that. There are accesories you can get to make that work better in a car.
If you just want to play MP3's, using a full blow carPC to do that is just plain overkill. People here use the carPC for a lot more than just that. That would be like buying an entire computer just to get a cd player. You could do it, but there are much less expenive ways.