Sometimes http://www.holidayautos.com has good deals.

If your flying into Treviso Airport, you could book a sixti car. 15 days are available from less than 200€ for a Fiat Panda (5 doors). However, they have a km limit, so you might take that into your calculation. I wouldn't care much about the signs on the car. Sooner or later you will be recognized as tourist anyway (signs you wouldn't think of, like which parking spot you've chosen etc). After all, Italy isn't that bad in terms of safety, especially the North.