There are lots of villas in Tuscany and many of them are hotels, some are set up solely for events, others for both. What do you have in mind? Just a day there, or sleeping too? How large does it have to be, for how many people?
This should help find something more specific - "a villa in Tuscany" is just too general. Give us ideas of what you'd like to do there and for how long, that way we can see which ones offer what you need.