If ou are long time visitor, you should have some ideas about Displays...
what About 7" widescreen Lilliput Display, it has VGA input, 2 RCA inputs, and if you want, Touchscreen. you can mount only the cables into your car, to connect removable Display and laptop (display from one end, laptop - from another end).
it is about 270$ on eBay (cheapest option) plus S&H (total about 300$), w/o touchscreen - 50$ cheaper.
As for removable variant, you can use some rails for laptop, and some stick (where the display will be on) for display, if you look at the back of these small displays, there is a mortise for it.. I plan to do removable solution for display, I have some ideas about it, when (or if) I done, I will show you.