I see biggest problem in...the LCD screen itself. In car it must have enough contrast at daylight and it must be easily readable (but not disturbing!) at night.
Everything else is a question of programming (skills). I'd preferr to write the code for some higher-end PIC (18xx series) and in assembly (to provide speed) but naturally it would be much easier and quicker to write in some high-level language. Say, C++ etc. This might increase amount of needed resources (memory, processing speed) but not necessarily.
Writing in PC-like languages (Delphi...) in my opinion demands using much better processors: ARM, may-be even a small PC.
Correct me if i am wrong.
On the other hand....using a small PC gives many additional features- displaying videos etc.
It's up to You!