Milan ..which airport?

Most trains to Florence leave from Milano Centrale train station. Getting to the train station is easy from either Malpensa Airport (direct Malpensa Express train takes 45-50 minutes) or from Linate Airport (direct Starfly bus takes 30 minutes).

Malpensa is the big Milan airport, a good distance out of town. International and longer Europe flights.
Linate is the smaller airport, closer in to town, regional and other shorter flights.

I was going to do my lease car in Nice, but flights are high. Flying into Milan is much cheaper but the lease car is more expensive. I guess its a wash. Milan is closer to Florence than Nice.

Consider that you will save the drop off charge if you rent and return the car in Italy rather than rent in France and return it in Italy.

Have you considered taking the high speed train to Florence then renting a car in Florence?

You won't need the car in Florence, but you can rent it after visiting Florence if you want to drive all over Tuscany.

it cost about $400 more in fees to lease a car in Italy vs France