Ciao Glitter,
If you're in a wheelchair, then of course buses are not really an option for long distance travel but they are for local travel. In Florence, all of the buses are equipped with platform elevators and a dedicated space within the bus for anyone traveling on a wheelchair.
Seeing as you'll have 2 weeks, I'd suggest breaking it up into 2 separate bases. You can spend the first week in Florence exploring the city and the nearby outskirts.
The second week you can use a base outside of Florence in the Tuscan countryside to explore the rest of Tuscany from there. Driving is not difficult, it does get a little bit of getting used to at first because Italian drivers are a bit "aggressive" but as long as you forget them and focus on driving, you'll be fine. A GPS is certainly recommended to aid in moving around. You will only really need a car once you leave Florence.
Another option is to rent a car with driver, but that can get pricey... but you could arrange it only for specific days you want to go out to specific destinations.
Having said this, you have to realize that most of Tuscany is made up of hills.... and that in the past centuries, the "safest" place to live was at the top of the hills so that you could have a good view onto the surrounding area and not be caught unawares by enemies. So most of the villages in Tuscany are set atop hills (some really gentle, some a little bit steeper) and therefore require some maneuvering with wheelchairs.
Florence is a great exception - most of it is on flat ground right on the bank of the Arno river.
I suppose your husband is not confined to a wheelchair and will be your main support for moving around. So Florence is a great base for your first week. Here are some
options for places to stay in Florence that offer disabled services and rooms.
The Chianti countryside and the area south of Siena are both also good options for "gentle hills" for your second week: take a look of some of the
options here.
If you have a disabled parking pass in your home country, bring it along as you qualify for free parking and free entrance into the ZTL area in Florence -
read details here.
Hope these suggestions help you get started on your planning! If you have more questions, do post and I'll try to help!