I have a Varizon cell phone. The phone is a Samsung SCH-A790 Series. I can switch from CDMA which is what we basically use in most of the Western Hemisphere. In Europe I switch to GSM Mode which is Intenational. I am a long time Varizon customer, so I was given a real cheap deal. Varizon stores don't usally carry them in stock, so you have to call the 800 number and they will ship it right to your house.
Pluses.
You don't have to switch SIM cards where ever you go.
Your loved ones and friends dial your normal US number as if you were at home. You phone rings where ever you are. and voila you have an incoming call. You dial from your phone just like you would dial from Europe. You have to dial 001, the Area Code, and Phone Number.
Big minus. It costs $1.29 per minute whether you call out or are called from the US.
However when you are in the CDMA mode at home, your normal rates apply.
You are better off buying a phone card for the important private secure calls.
I am based in Europe and I bought a cheap NOKIA phone under the Orange Phone Plan. I have the pay as you go plan. I top off the phone every month. It cost me 35 cents per minute to call the US. There is no charge when people call me on the NOKIA. Better deal than Varizon.
Sounds like you will be travelling to different destinations. The Orange Plan would not work for you.
Go with the phone card.
As for GPS's look on EBAY, or go to an avionics shop and see if they have used hand helds.