Ciao DeniseB,
Yes, I think the plan is ambitious if you were planning to visit Montepulciano and drive back up. But since you're planning to move your base down to Montepulciano and then stay there for 5 days, then you can certainly plan on taking the whole day to drive down through Chianti, stopping and visiting the towns as you drive down.
If you head off in the morning, I'd recommend a brief stop in
Greve in Chianti. You can visit several wineries in the area, several located right before you actually arrive into Greve. Not sure how early you want to start on the tastings....
One small, family owned winery we recommend (also offers apartments) is Fattoria Viticcio. You'll need to book ahead of time to see the cellars and do wine tastings. They're open from 9 a.m.-noon; 2 p.m.-5 p.m. Monday through Friday. Saturday and Sunday closed unless booking arranged beforehand. You can contact them through this page:
Fattoria Viticcio
Then keep on heading down toward Panzano. You could have a nice lunch here at one of the trattorias near the main square. A very popular place if you like meat is to eat at the restaurant by the famous town butcher, Dario Cecchini. I've heard good things about it although I've still to go and try it myself. During the week for lunch, this menu is on offer:
Dario Doc
There are also several wineries along the road before and after Panzano where you can stop at and do wine tastings as well as buy wine. Winery tours generally require bookings ahead of time so if you want to stop and visit a cellar, look at this list and book at one:
Wineries in Panzano
Continue on to
Castellina in Chianti where I recommend you stop and visit the town, taking a walk around the tunnel built into part of the walls.
From Castellina I recommend you continue south toward Siena but I particularly find the road toward Castellina Scalo pretty magnificent with all of the vineyards. If it isn't too late in the day, you can stop at
Monteriggioni before you head onto the Firenze-Siena road to head around Siena and Montepulciano (while you're staying in Montepulciano you can head back for a day in Siena if you haven't seen it yet by then).
As you can see, this can be a whole day and you'll reach Montepulciano late... so be flexible and decide where to stop, maybe some places you can just drive by and enjoy the scenery. Depends why when you need to be in Montepulciano, if you want to be there by dinner or enjoy dinner along the way. You'll have a great time!
