If there is an airport in Florence, it would be very small. You're best bet is to take a flight to Fiumincino (Da Vinci) and then take a train or bus to Florence. Either way, it's about 3 hours. If there is an airport in Florence, you'd probably have to go through Rome anyway.
By the way, trains in Italy, assuming there is no strike (sciopero), are always on time.
Sheryl