Your project sounds very similar to what I started last week! ;D
I'm Hooking my Arduino up to my old Asus eeePC 701. The 7" screen should be about the right height for my car's dash, and leaves some space for any dials I don't present on the screen.
Since the Arduino Editor is written in Java, I can include the same libraries then write the rest of the display in normal Java. I should need very little processing power on the board itself as I have the eee's Celeron to convert sensor values (0-1023) into real numbers for display.
I haven't considered the input voltage of the laptop yet, but as it requires 9.5V, a DC-DC transformer should be reasonably efficient and easy to find. The laptop's battery will also provide protection from sudden shutdowns etc.