A bit more hunting around at lunch time and some reading of the MP3Car.com website and It would appear you can get a 250W PSU that will power a shuttle:
http://linitx.com/viewproduct.php?prodid=12004
http://eu.shuttle.com/en/desktopdefault ... ead-14213/
So my current thinking is a shuttle motherboard as that will provide 3 internal SATA ports, 2 external SATA ports, 4 USB ports, 1 firewire port, an Optical S/PDIF output, integrated VGA support and a wireless LAN option. It is also a small form factor and doesn't need a big cooling setup.
It also has the added advantage of using standard PC hardware enabling me to plug in any bits and bobs that I have lying around as well.