I use a laptop
Search for a car adaptor that will work with the laptop (cigarette type) you can then locate a cigarette power sourse near by (eg boot/trunk) thats easy to do and cheap. (One that only powers when the car is on AC/Ignition)
You can use a software shut down program, that will use the battery after power is cut, and then hibernate after a set time limit.
As for starting the laptop you can buy a kit that will solder to the laptops power button, and simulate the power button being pressed when the car starts.
Other way is to go for the Carintex range, which involves alot more wiring...