Right, I have decided to built one. It seems to me there are a few 7" Touchscreens floating on tardme for $360 buy now. Unless anyone here can do them cheaper.
So the PC with micro ATX Mother board some el-cheapo CPU will cost me about $300, I will use inverter to power it (works out cheaper than buying 12v to 12v PSUs) and some custom case. I will chuck it in the boot somewhere. The box will work out about 250mmx300mmx100mm. which is relatively slim. I will think of the way mounting the screen into the double din space (should not be a majour). Also I am thinking of some relay box for power to have ability of soft shutdown on key power off.
My system will be mainly for music (not interested in movies), I might add some GPS capability later on, so I do not need powerfull CPU (infact lowest on the market will do more than enough).
I will have to think how to suspend the HDD so it will not fail prematurely.