If my understanding is correct, you can only buy the regionale tickets a week in advance. The prices do not change so there's no advantage to buying them early, EXCEPT that it's better to have them in hand when you get to the station, so you don't end up missing a train because you are waiting in a line to buy tickets. You can buy your tickets to Assisi anytime while you're in Florence, the train station is very close (the main TI is across the street from it) and the ticket lines can be long. Assisi is a small station, and the Tabacchi (kiosk) there sells the train tickets. I'd buy them when you arrive in Assisi if you don't buy them in Florence. While the kiosk isn't generally busy, you don't want to be waiting while there's a customer ahead of you with a lengthy purchase. Also, do NOT forget to validate your regionale tickets in one of the machines on the train platforms just before boarding. If you don't see one, just hold your ticket up and ask anyone "dov-eh?" (where). They'll know exactly what you want.
You will have to buy your tickets to Sorrento when you get to Naples. The Circumvesuviana is in a separate station below the train station. That's where you buy the tickets. There's more than one train route, so be sure when you board the train, that it's going to Sorrento - last stop on the line. If it's hard for you to go down steps with your luggage, there's an elevator down to street level in Sorrento. You'll go down a few steps and see the ticket agents on your right. Turn left to get to the elevator.
