I am not sure why you would drive to Florence from Venice instead of taking the much faster high speed trains. You are not going to need a car in Venice or Florence, both are 'verboten' to cars.
From Venice Santa Lucia station right on the Grand Canal, you can be in Florence in 2h 5min by train. Then from the Florence station (Firenze Santa Maria Novella station) you can take a taxi and be at the Silla in 10 min.
I presume you will be in the city center of Venice. That is over 1 hr to get to the Venice airport (actually it would make more sense to pick up the car at the Piazzale Roma location). Then you need to do the paperwork for the rental. The drive to Florence is 3.5 hours on a good day. Then you have to find your way from the freeway exit to the hotel Silla (20 min with me at the wheel, and I'm from Florence).
Anyhow, I hope I convinced you to drop your car plans and take the train instead. If so the websites of the 2 train companies are here:
www.trenitalia.com
www.italotreno.it
If you still think you need the car, then you need to follow directions first to Padova, then to Bologna, lastly to Firenze. Once you come down the mountains into Florence on the A1 freeway there will be 4 Florence exits: Firenze Nord, Firenze Scandicci, Firenze Impruneta, and lastly Firenze Sud. Firenze Sud is the exit you want, the last one. Pay toll and go straight. You are on Viale Marco Polo. You can exit at Viale Europa (which I would do) or across the bridge on the Arno. Then work your way to the city center (have a GPS with you).
The Hotel Silla is on Via de' Renai. That is sector F. Sector F is not in the ZTL during the day, so if you arrive during the day you can enter coming from Piazza Ferrucci. Go straight west along Lungarno Serristori, then turn left at Piazza Demidoff (it's a rectangular piazza with trees). During summer, sector F becomes part of the ZTL in the evenings only (lots of bars in the area) on Thu/Fri/Sat. There is no camera there, unless installed recently, but at night there will be policemen at Piazza Ferrucci.
Edit: There is a camera now. I saw it on Google Street View.
So if you drive on Thu/Fri/Sat make sure you arrive before the evening. The ZTL starts at 11pm on those nights.
