Easier to buy components.. they don't have to be all individual (like ram, processor, case, motherboard, etc etc), but do seperate larger "chunks" of things.. for example:
1) complete PC
2) touchscreen
3) PSU (Opus most likely, or PC might have one already)
..that would probably about cover it, minus whatever cables you need to extend the things to wherever you're running it in the car.
EDIT: I also have a laptop for a CarPC in my sig, plus a monitor, no PSU needed only an inverter..
-arebelspy