that time? which time?

usually the South of Italy is famous for the sun...

you can check here tha climate for nov in all the Italian Region

Sicily: http://www.ilmeteo.it/portale/medie-climatiche/Sicilia
Campania (Naples, Sorrento, ...): http://www.ilmeteo.it/portale/medie-climatiche/Campania
Calabria: http://www.ilmeteo.it/portale/medie-climatiche/Calabria

check the Min/Max degrees in that month ("Temperature Minime/Massime") and the mm of rainings ("Precipitazioni")

of course, it's best to book in advance, so yo can get a wider offer of accommodations, including B&Bs;
cheap accommodations (and expecially Bed and Breakfasts) tends to be sold out soon, because they have just few rooms, unlike the hotels.

for sure it's cheaper to stay outside venice.
But I don't think it's so much better to take a train from Florence to get to Venice.

Probably you can take an hotel outside Venice, in the inland (for example in Venice Mestre or Mogliano Veneto) and get to Venice from there: Florence is too far for an escapade to Venice