Use a netbook, they are el cheapo, you don't really need the processing power. d510 based one are ~$200-300. Comes with everything you will need.
As NSFW said....power management, power management, power management. Rigging up on off is a hassle, and tapping into ACC or IGN, using DC to DC off battery etc.......
netbooks, with in power, let it never sleep, and even shut of the minitor if needed, cat turns off, goes on internal battery, after 5 minutes, have it sleep. Set to wake on usb, ie touch the screen....ready to use under 5~6 secs.