A variety of prebuilt ones are available. Check out the mp3car.com store, they have prebuilt units available, and they can even install the OS and test things before shipping (for more $$$)
Backup camera would probably go right to the screen, bypassing the PC altogether. However there is nothing stopping you from using a video input to the PC (like a PCI video input card) and recording the camera or whatever else you want. Someone did liscense plate recognition I think, or speed sign recognition.
As for the fabrication, Nexon (or Nexson, I forget his username here) does custom fiberglass work with great results. Again, expect $$$.