I know this thread is a tiiiny bit old, but to chime in on the GPS and Navi. While me and my friends stayed in Takayama, we rented a car from the local Toyota rent-a-car. One of the employees spoke English and offered to set up our Navi system for English and even programmed our destination.

We were heading to Ogimachi Village in Shirakawa-go: http://www.japan-guide.com/e/e5951.html and got there no problems (it was a 90 minute drive or so?). I guess it depends which place you go to and whether the staff may be able to help you.