Car rentals are easy to obtain. If you take the train from Venice to Florence, there are car rental offices within 500 meters of Santa Maria Novella station. I know Hertz and AutoEurope have offices on Via Maso Finiguerra. They are also available at the Florence airport.
We use the car while driving around Tuscany then drop it off. We've dropped it off at Fiumicino Airport because it's easy to drive to and no real inner city traffic. We then take the train or taxi into Rome.
This year we're dropping the car off at Orvieto and catching the train to Naples. Good Luck. Have fun!