I don't know La Fattoria but I know Sarteano pretty well, I was there just this morning for the market. It's a marvellous little place with a superbly unspoilt old town that sees shamefully few visitors. There are some good restaurants too. It's well placed for getting around as it's close to the A1 at Chiusi and, of course, the station if you want to train into Florence. Obvious must-sees in the immediate area are next door Cetona, San Casciano dei Bagni (approached via a gorgeous road cutting through the lower slopes of Monte Cetona) and, going the other way Chiusi Citta and next door Citta della Pieve. Orvieto too isn't far. Montepulciano is only 15 mins away, the gardens at La Foce (open on a Wednesday afternoon) a similar distance. Maybe drive up to pretty Castiglioncello del Trinoro for coffee (or aperitives) at the new hotel up there. From here, there's a fabulous drive down a dirt track into the Val d'Orcia from where it's easy to get over to Pienza, San Quirico, Montalcino.
Easiest to rent cars in Chiusi via AutoEurope.com . 12 people to Chiusi where there is no ZTL to confuse you when you go to car pick up. Hertz,Avis,Europcar are walkable from the station. PS: a vehicle which holds 12 people and a driver is a mini bus which you DO NOT want to rent