Dash options:
1. briefcase computer, mount it. - cheap and durable
2. laptop, mount it. - looks nice and saves space, but expensive
backseat/trunk options:
1. regular computer with REALLY sturdy case, mount it. - cheap but very fragile
2. briefcase computer, mount it. - cheap, durable, no one sees it most likely so it can be big and really powerful liek a regular desktop.
For a car you REALLY want something durable than can get stepped on, dropped, anything. I suggest a very thick briefcase, possibly steel or something really tough. if its in the dash then probably not possible as there big.