Not quite sure what you mean about the different options for the computers. Most people use one of two options, either they build themselves a really small pc using a mini-itx board (www.via.com.tw) or they just use an old system they have lying around the house. Both choices have advantages and disadvantages (mainly cost and space required).
From what I've read most people don't worry too much about the DVD-ROM drive, generally if you get a big enough hard disk you can store most of the movies you want in Divx format. I think a couple of people have extended the length of an IDE cable using some CAT-5 (not 100% on that) thus allowing them to mount the DVD drive in the dash. Generally if you opt for the mini-itx option then you can mount the system somewhere up the front of the car, either the glove box or under the passenger seat etc. This allows the DVD drive to be accessed from the drivers seat. Basically you need to have a little inginuity when planning where to mount your system.
What car are you planning on mounting it in?
Anyway, hopefully this helps you out.