Sounds like you're asking for a general purpose cpu... How about this:

1: Emebeded system board with good expansion and sound capabilities. They pretty much exist off the shelf right now for about $300. The only problem is most of them don't have very good sound on them.. but some do.
2: Dash display module (vfd, oled, whatever) with buttons and ir/bluetooth reciever. (both are dirt cheap)
3: Emplode style software with open api for adding things like obd/gps/etc/etc.
4: A nice chasis for trunk mounting and holding expanion modules like dvd/gps reciever/802.11/etc/etc.
5: A smart way to get stuff on to it (hard drive module with home docking sled? I dunno)

So I guess, a computer with a good remote display, chasis, and software. You could probobly build the unit for less than $600 with no storage or add ons.