From Venice, you can take trains via Bologna and Florence to Chiusi - that's the nearest train station to San Casciano in Bagni (about 25km away).
From there, the easiest solution is really to rent a car --- also because even if you hire a private driver to pick you up in Chiusi and take you the last part, how are you going to move around once you're in San Casciano dei Bagni? Have you mapped it on Google Maps, it really is in the middle of the countryside along the Tuscany/Umbria border.