You really won't need a GPS in Yucatan (or Quintana Roo). Almost everyone in the tourist zone around Cancun speaks English (though they might want you to think they don't.)
We've been to Cancun several times and THE worst part (by far) is getting out of the airport! Just think of yourself as shark bait: Know exactly what you're looking for and the exact address; only talk to airport personnel; get your gear and GET OUT.
When you pay for stuff (especially at the gas station) make a point of telling the clerk the amount of the bills you're handing them (as in $... out of 500 pesos.)
Don't speed. And don't drive drunk. The cops are not there to be friendly.
Make sure you go to the Mayan ruins (Tulum is just for tourists). Muyil is only a few extra miles down the road and almost undiscovered. Coba is a good choice if you want/need more services.