Get the International Drivers Permit. Read up on driving in Italy.
A better solution to the rental GPS is to take your own. We have a Garmin that accepts SD cards with maps. We bought the Italy/Greece map. The advantage to taking your own is that you can pre-program addresses before you leave for your trip. As soon as we picked up our rental at Rome FCO, I plugged in the Garmin and went to Saved Places and pulled up our agriturismo that was just outside Greve. The Garmin took us directly to it!
I think the ZTL situation tends to be overblown. If you are in small towns, you'll find ZTLs well marked and easily avoided. If you are visiting a small hill town like Radda or San Gimignano or Volterra, the rule of thumb is anything within the walls is in the ZTL. There's parking outside the ZTL, you just park and walk in.
ZTLs in larger cities can be trickier, especially in Florence and to a lesser extent in Siena. We had an easy solution for Florence, we just left the car behind and took the bus into town. Since we were near Greve, we just parked in the free municipal lot.
When driving you should make sure you scrupulously adhere to speed limits. There are automated speed checks, including speed cameras. Also be aware that the legal limit is lower than the US, .05 as opposed to .08 for the states. For at least one visit to a winery, do a food and wine pairing at lunch or dinner. This is the best way to taste and appreciate wines.
Be sure to drive the Chiantigiana road. It is spectacular! We also took a drive from Greve to San Gimigano/Volterra. That's a good day trip. How many days in Tuscany will you have?
We had a great time driving around Tuscany. We based ourselves at an agriturismo and spent our visit doing day trips. Study up and follow some common sense guidelines you you'll be fine.