This is regarding touchscreen capability:
Any ideas for help?
Now talking about actual display, it all depends on two factors:
a) does an aftermarket dvd unit of your choice have AV input connectivity?
b) does you carputer motherboard/graphics card have TV out?
if both answers are "yes", then you can connect your carputer to your DVD unit and use it for viewing carpc output.
But the question stands: Is it worth it? The answer is IMHO "NO".
To find out the reason ask yourself a question: Why would I want to spend 300+ USD for DVD unit and 500+ USD for carpc if:
a) its one of the most wanted features of carpc to play DVDs
b) you can get 7,2" TFT PAL/NTSC display for 100+ USD. Such screen will output picture of same or very similar quality (understand: very crappy quality) as the one found in your DVD player