1.Are you familar with the linux OS? If not, i dont think that would bew the best route.
2. There are scripts that you can run for auto logon, i think mandrake linux allows auto logon and auto X-Server startup.
3.As far as touchscreen modules you would most likely have to rebuild the kernel (refer to 1) with HID and touchscreen support and its a pain in the @ss to configure.
4. If this would be your first encouter with linux i dont think this would be the setup for you, besides the Reiser filesystem, (here comes the debates) linux is not ready for in car use. some might find it usefull.But as for now Windows XP Pro would be the best way to go, sorry for any letdowns.
I think a linux based frontend in Linux would be awsome but no real eyecatching ones