Get a developer board such as a Freescale board or a beagleboard or something else that will run Android and use one of their camera setups and you would be all setup.
Otherwise trying to get an external camera working with a standard tablet etc could be difficult.
You can get one of these development boards with a touch screen for about the price of a decent tablet.
I have a couple of these boards here:
You may be able to use a standard backup camera with a USB decoder on some of these boards if you do your research. Otherwise you could rig up one of their cameras.
Since they are development boards you are more likely to load them with additional drivers...
But in the end you are not going to save much over the cost of a cheap PC.... And much more headaches.