1. Any laptop w/ DVD capabilites will be around $700. Or you can get a slim notebook w/external DVD option (Sony) to avoid the extra bulk and use only when necessary.

2. Sound output via the tape converter is an excellent idea! the sound quality is based on the quality of the tape head, so if it sounds good to you, use it.

3. Your GPS system would work just fine, as long as the maps are from CD, resident on the laptop, anything that doesn't require internet connectivity.

4. Lastly, you can probably find a body shop that outfits police cars for the laptop stand. Something within reasonable view (or you can build one yourself) and collapsable (do you have passengers)?

5. Outfit the car with power inverter, or get a DC charger version for the laptop. Used laptops generally have a horrible battery and/or the DVDs suck it dry real fast (2 hours).

Piece of cake project (less than a day) once all of the materials are in.

Hope I didn't forget anything.

Hell, to make things more interesting, put in some speech recognition, write some automation scripts, and mount a mic to the laptop for verbose control of the laptop.