Florence Car hire

Oh please rethink this! You have no idea what driving is like in Italy until you actually experience it, and I say this as someone who has done it (also a wrong side of the road driver)! Roads can be very narrow, busy or ZTLs where you can’t drive at all and will be fined for doing so. You’re used to driving on the other side of the road, and although it’s not hugely difficult to make the change, it is a distraction that you have to concentrate on. And most importantly, Italian drivers have a whole different driving culture that is very different to anywhere else - they have different ideas of what is expected behaviour and that can make for confusing and dangerous driving to people who aren’t used to this.

If you need a car to drive around Tuscany, please consider taking the train/bus to Siena and pick up your car from there when you leave it. it’s a much smaller town, you’d pick up the car from the outskirts and wouldn’t have to drive through the town. And research ZTLs so that you don’t pick up fines that you on,y find out about months later. Good luck!

Driving around rural Tuscany was a pleasure. Driving in city traffic (Florence in particular) or on the Autostrada was stressful and generally not much fun for me as the driver. In retrospect I wouldn't have wanted my introduction to Italian driving to have begun in the city.
Since you won't need a car in Siena, and you'll already be at the Florence train station anyway, I too suggest continuing your journey that day by train to Siena and renting your car there once you're ready to commence the driving part of your adventure. It'll make for a much easier transition to driving in Italy, not to mention negotiating the learning curve of driving on the other side of the road.
We found the roads in Tuscany to be in excellent condition and the driving itself was pleasant. There is a learning curve to be negotiated however, and learning how to recognize and avoid the dreaded ZTL's is part of the learning process. That said - it's really not that big a deal. If your daughter is a competent driver at home in Oz she'll do fine in Italy.
Mapping apps like Google maps or Waze worked great and made navigation a breeze for us. In particular, using the street view feature on Google maps was extremely helpful for me in previewing our daily excursions.
If you're up for some off-the-beaten-path drives while in the region I can recommend the guide book "Back Roads Northern Italy" from DK publishing. You can usually find used copies for just a few dollars on Amazon. We wore our copy out during a 3 week drive around Tuscany and Umbria a few years ago.

Edited to add: if you do elect to rent your vehicle in Siena check the operating hours of the various companies - some can be pretty restrictive with weekend closures for example.
Consider too the insurance options for the vehicle. Most credit cards (American types anyway) specifically exclude Italy in their coverage, though some of the premium cards do. Many of us here rely on AutoEurope for rentals in Europe, and in Italy in particular, due to their competitive pricing and clear explanation of the various insurance options.

We rented from Europcar in Florence. Their office was an easy walk from the train station. They were very busy, but also great to work with. The drive out of Florence was a bit tricky but not too bad. We drove south towards Siena via Chianti.