I cannot imagine winging a vacation to Italy in July without rooms booked. Maybe just me but I like to have a choice of place to stay and read reviews before booking, you will be taking last available and in some cases driving from place to place inquiring I would think if you waited until you were there.
Would recommend taking a look and pricing at
AutoEurope.com
(they are actually in Maine and a consolidator much like Gemut)
Have never used Gemut but personally don't like that there is no way to get a quote easily within submitting a form and waiting for someone to get back to you.
Some good tips things to be concerned with mentioned already in this thread I will add that the well known US brands that exist in Italy are just licenses to use the name, they are not connected with the US company other than by name.
Most advise is that you want to stick with Hertz, Avis and EuropCar ; those are the big 3 and often who a consolidator like Gemut or AutoEurope will choose, but not always ; the others some of which are well respected here in the US have very poor reviews in Italy.
AutoEurope also has a deal where if you rent a car through them they offer a GPS unit for very little cost that is preloaded with the maps you need. The cost basically covers the shipping so the unit rental is virtually free, as they send the actual unit to your US address the week before you leave the US and then when you return you send it back to them. You have to ask for this it is not advertised but a fairly painless process.
The fact that most towns have a pedestrian zone, many are surrounded by walls and have ZTL areas makes hopping from town to town with a car to see the sights much less convenient than it is say in the US.
In general though, as long as you stay away from trying to drive in the big cities it will not be bad and the best way to see the countryside.