By 9 pax van I hope you are including the driver (8+1) because if the driver is not included you need a minibus license (type D1 in Europe), for up to 16 passengers or a D type (for any bus without pax limits).
Check rates on:
www.autoeurope.com
If you have 2 or 3 people willing and able to drive, 3 cars are often cheaper than a van. Italian streets are narrower, especially in ancient historical centers. 2 lane highways outside of cities are also narrower than in America (sometimes even narrower than the min by law of 2.8m per lane, or 9ft wide) and there is no shoulder, so don't scrape walls on the sides.
I confirm that in Italy you will need an International Driving Permit (IDP) which you can get at any AAA office for $29. Although rarely asked at the rental car desk, if you get pulled over (random police checks are very common in Italy, even if you don't commit infractions) there is a minimum fine of 408 euro for driving without an IDP.