If you want GPS i'd get a 6inch min screen. Anything under that you will have a hard time seeing. My friend tried it on a 5inch and it was doable but not pretty.
I've seen gps software that will show the map with directions along with tell you when to turn via the sound card. A very nice option so you don't have to take your eyes off the road. I'm not sure if any computer software will do that since I haven't looked into it.
As for what to look for. Go with a VGA screen if you want to spend at least 300 for the screen and the controller. If you want to get a RCA input screen then you can spend a min of 100 up to 300. The one thing to look for in any screen is the brightness and this will be messures in nits. Look for anything around 200. A lot of cheap screens will be around 70 i think? and they suck in direct sunlight.
If you go with a VGA screen you use a controller card if you go the other option its a tvout card.
As for theft, I have mine on a arm that i can just slide off and place it into the center console when I leave my truck. I only use it on long trips anyway. As for sun.. just keep it out of direct sunlight.. there are many ways to do this.