Another thing about car-puters is, you need to use a daylight readable monitor. Or locate the monitor in such a way as to have it shaded. If you've ever tried to use a lappy in the daylight, you'll know how hard it can be to see the monitor in strong light.
Another thing to look at (if you live in colder climates) is the monitors minimum operating temperature. Sometimes it gets well below zero F here at night during the winter. A monitor thats been sitting at -10F all night might not image properly until it warms up a bit.