You have picked a wonderful place to go. The magic of Tuscany has captured us - heart &soul. Your choice of three nights in two sections of Tuscany is a wise one. The driving from the Chianti region to Montepulciano, although magnificent visually, is along twisting hilltop roads and even though it looks close on the map, it is a long way. I am sure you realize that you have picked the best wine areas - Chianti, Vino Nobile di Montepulciano and Brunello di Montalcino. If you are interested in a private guide for these regions, I will give you the name of someone who will make this area come alive. A day with Dario will not soon be forgotten.
Rather than stay in Monteriggioni, I would suggest you stay right in the heart of Chianti country - say around Radda or Castellina in Chianti. I can recommend a few places if you are interested. Also, we spent a delightful few days last September in Montefollonico at La Costa at a rate of 250,000 lira. The hotel was in a small village, a short drive to Montepulciano and all wonderful points west such as Pienza, Montalcino, Sant' Antimo. It was recommended by a fellow ziner, Bill Sutherland, and we enjoyed it immensely. We have stayed in a small and basic apartment just outside of Radda on an active Chianti vineyard many times. The rentals are week to week and the price is about 750,000 Lira per week. If you decide to stay in one spot, this might work for you.
We have stayed at the Hotel Belvedere in Bellagio and found it to be fine, although somewhat weary at the end of the season. It was a short walk to the centre of town - uphill on the return. The views of the Lake from our room were breathtaking. Don't hesitate to stay here.
I agree with Deb - Porto Roca is the place to stay in Monterosso al Mare. The crazy cab ride (compulsory) from the parking lot to the hotel is worth the price of admission alone. The breakfast terrace, high on a hill overlooking the sea, is beautiful.
The Hermitage is our favorite hotel in Florence. Although pricey - it offers a wonderful location - along the Arno just at the Ponte Vecchio. Ask for a room on an upper floor overlooking the Arno. The soundproofing enables a quiet stay. It also has a charming breakfast terrace on the roof with views of the Arno in one direction and the Duomo and Campanile in another.
Having spent many weeks in Tuscany throughout the years, I have amassed copious notes and would be delighted to share any information that may be of use to you.
Have a wonderful time
Felice Toronto, Ontario, Canada