if you want to do GPS.. then I recommand a vga screen. These will run you around 400 for the screen.
Now powering is easy. You can get a dc-dc PSU those are around 150. The run right from your battery. Or get an inverter. Now i guess you've seen those inverters with the cig lighter attachments. You can either get one that is made to wire into your car batter or just cut off the cig lighter and run the wires to your battery.