1. Please see here. There is an entire FAQ section dedicated to helping users understand power suplies, powering PCs in the car, picking out power supplies, etc.
2. The Touch Screen is generally used in place of a mouse, yes. There are double click utilities (usually, tap the screen and hold your finger on it till the right-click menu comes up).
3. Again, follow the link above to find information that will be helpful to figuring out what size power supply you need.
4. I don't know where you heard that using x size screen would have anything to do with if it would freeze or not. If it were such a problem, I don't think there would be so many people with screens in their cars.
to the forums.