It seems daunting, but it really isn't too hard...
mp3car.com is a good place to suss out some gear, but when it comes down to the actual computer, go to a local independent computer store to build the PC.
Cost of computer will vary on what it is you want to be able to do.
You stated that you wanted to watch DVD's watch TV, listen to radio, listen to music etc etc. Well an excellant program that can help with that is Windows Media Center, it comes free with Vista ultimate, or stand alone.
Ultimatly i think that WMC is the best software for controling you're entertainment..
As far as GPS and Bluetooth and internet is concerned, you can use plain old windows for that, bundled with a GPS reciever, bluetooth reciever and WiFi card or wireless internet card and other software - most of wich is free, you can have a great system..
Then comes the tricky bits..
1. finding a place to hide it all.
2. Working out how to supply power to it.
3. working out how to get sound from computer to you're speakers with best quality possable for you're budget.
Basically you're looking between $700 - $1300 depending on how much gadgets you want. (not including speakers, sub, apms etc)
I'm about to start my project on this very soon.
eBay US is you're friend, theres plenty of good carputers already put together, all you gotta do it wire them up..
Hope this helps, and sorry for my paragraphing and bad wording - i'm too tired.