I did the Florence/Tuscany 2 week untour last year and it was fabulous. My advice - and I mean this with the utmost sincerity - is to change from Buonconvento to Montalcino, which is 10-15 minutes further, but seems to me so much nicer. We drove through Buonconvento a number of times on our way to other towns and it's a very plain compared to Montalcino - which is like a fairy tale. Just my two cents but if you still have time to switch I'd think about it. Hope this is not too direct and that the info helps.
We went to Buonconvento last Fall. We stayed in the Donella apartment. We were also a bit apprehensive about driving, but with the aid of a GPS, maps and a cheat sheet on the road signs we managed well. You will have to watch out for fast Italian drivers. We were clipped by one who tried to pass us on our way home one day. Fortunately no one was hurt and the carbineri were on the scene almost immediately. Even though none of the Italians spoke English and my Italian was very limited we managed to communicate and were assured that the other driver assumed responsibility. I also called our Untours contact, Harriet, while still at the scene. She was most helpful and all ended well. We were able to continue our vacation with peace of mind.
The little hill towns are fascinating. The countryside is beautiful. The food is delicious. Our hosts were gracious and invited us to dine with them twice. We even helped with the grape harvest. By the way Buonconvento has its own charm and a rail station from which you can travel to Florence, etc.. While we were there there was a festival, market day, and wine judging in Buonconvento. We also found Montalcino delightful and went there twice. I could probably write a book about our 2 weeks. It is a magical place.
The driving in and around Buonconvento is not difficult. Italian drivers do drive a bit more aggressively than Americans, but not nearly as aggressive as the French. They will pass in places that you might frown at, but let them in and there's no problem. Just keep your wits about you and you'll be fine. If you're not experienced with a standard shift car, I do suggest an automatic. It'll be worth the extra cost.
Remember that you should park outside most Italian towns because there is very little space available in the towns. I suggest that if you drive to Florence, park outside the city and take a bus into town.
We stayed at le chiuse 4 near Buonconvento last summer and had no problem with driving. You simply drive at your own pace and let the Italian drivers pass you. Nothing personal with them, it is just their way. I would not drive into the larger towns like Siena or Florence but the smaller towns are OK. Just make sure you pay for parking at the kiosks.
We did the Tuscany Untour last summer and driving is no problem. Just make sure you know where your GPS is going! The only problem we had was, when we were going home, we forgot to check that it was going to OUR home, not the last person's who used it! It made for an interesting tour around the Tuscan countryside. Have fun!
When you park in the "free" parking lots, (white lines), they are usually for a specific amount of time - like one hour. Then you have to set the little cardboard clocks on your dash or window with the time you are leaving the car. We didn't know about this and went to the grocery store for 20 minutes and got a 39 Euro ticket. The blue lined parking is a pay lot and has a kiosk where you pay for how long you will be gone and then display your ticket on the dash of your car. That part we knew. Don't park on the yellow lined places. That's for residents only. Who knew parking could be so complicated!