I'd go for one of the sat nav apps you can get for a smartphone. I was out in a van with a driver the other day and he had the Co-Pilot app on his. Had it for quite a few years and loves it. I was impressed. Cheaper than a separate device and saves taking two devices with you. Get a good car mount and you're done.
As for the power source if you haven't got one any decent auto-electrician should be able to fit a USB port.
