Buongiorno,
They just added a new parking lot which will have you skipping ALL KINDS of city traffic. From Montecatini you will drive to the Autostrada and head in the direction of ROMA - then get off at exit Villa Costanza:
Leave the car (without leaving the highway) and take the tram: 23 minutes to Santa Maria Novella, every 3-4 minutes. You pay with telepass or with a ticket inside. Costs, 7 euros per day or 0.5 euro half an hour, 2 euros up to 4 hours, 4 euros up to 8.
I wouldn't try with the train because the train between Pistoia and Montecantini (so halfway between Florence and Montecatini) is under repair next week and they have substituted it with a bus...so it gets a bit complicated.
Buon Viaggio,
Donna Denise