GPS: very expensive to hire a car with a GPS IME. We bought a Miss Garmin in Monaco with 45 European countries pre-loaded for ~€150. Since returning to Aus we have added Aus & NZ, Japan and Indonesia. It has an micro SD card slot so it has more than enough room for the rest of the world. My advice: don't waste your money on hiring a GPS with the car. Buy one in Europe and take it with you on your future travels.
As to where to hire a car: we have used Argus car hire on 5 trips: Ireland, France x 2, Czech republic, Japan; no problems at all and good rates. May I suggest you get a quote from Argus and if you find a better quote from another company, take it.