Could be a good start? Doesn't look much better than the average headunit amp at 12v?
Or something like
I'm trying a hifiemediy usb-spidif, but there are loads around, same for power options.
I'll have a hunt through chinatobbys shop later for a screen.
Leaves software, which we might get away with one of the android car pc skins, or maybe try and redo xbmc? Maybe theres something already out there for a phone?
And buttons/surround. These would have to be outsourced, probably the chinese, but it could be done. Bet there would be a MOQ, moulds etc, which is where the investment would be I guess. Would there be any interest?