We rented a car at the Florence airport to tour Tuscany for four days. I priced a number of options for a 7-person mini-van and the best price was through Auto Europe and the rental company was Hertz. I have rented with Auto Europe in France and the UK prior to that trip. What I like about Auto Europe is not only their price, but customer service. I needed to change dates on my Tuscany rental. When I went online to make the change, the price changed—one call and I got the original price. They are also great if you have an issue. When we had an accident in France, it costs us just 350 Euros which was frankly a fraction of the actual damage. They handled everything quickly and on the spot, so no disruption to our trip.
As for driving in Tuscany, we spent most of the time in the Chianti region. The largest city we drove into was Sienna and the only real problem we had was finding parking for a 7-person van on market day. The roads were well marked, it was easy to get petrol, all and all one of our easier driving experiences in Europe.
One last note, having a car is great to explore the smaller towns.