which Venice train station? and where to stay?

The train station you'll want is Venezia S. Lucia.

You don't necessarily need to stay in a hotel near the train station, unless you desire to do so.
It makes sense to choose a hotel located near a vaporetto (water bus) stop & therefore you won't have a long way to walk.
Keep in mind that there are many bridges, which means many stairs to climb up & down, so it's best to pack light.
There are several vaporetti that travel to/from the train station. It's difficult to determine the time it will take to travel to the station because this would depend on where your hotel is located in relation to the station.

You may want to book your train tickets ( Venice to Florence) in advance at considerable savings here

The train station in Florence is; Firenze S. M. Novella.

Have a wonderful trip!

EDIT: Check this website for information about the Venice vaporetto.

Remember that you will go between station and hotel twice, but between hotel and attractions many more times.

Going to Florence there is no need to start extremely early (as you would if you were catching an early plane), so you have time to take the vaporetto to the station. Unless you love crowds I suggest you stay east of San Marco to avoid people going from the bus station to San Marco - you have to see those crowds to believe them.

any hotel recommendations that are close to tourist sites in Venice?

also, once we reach florence, we wont stay there but rent a car and go to tuscan countryside where we plan to stay (montelpulciano to be specific). Should we get a car rental at florence train station? or some other location?

With only 2 nights in Venice I would book a hotel close to the Santa Lucia station so you can be rid of your luggage as quickly as possible on arrival. Three star Hotel Abbazia is well located and well reviewed. http://abbaziahotel.com/
You have vaporetto access right there to St Marks Square.

Hotel Santa Lucia is a nice simple little hotel close to the station.

We have stayed twice at Hotel Ca'Nigra Lagoon Resort across the canal from the train station. Absolutely would do so again! They have a lovely rose garden right on the canal where you can have a drink with bar service. Their website says which canal stop to use, but if you are sturdy you can just go across the canal on the bridge (with many steps up then down) just outside the station. Google map it; a bit twisty, but not far at all.
http://www.hotelcanigra.com/en/hotel-venice/38-0.html

As you're planning to rent a car, I assume you're aware of the usual caveats regarding driving in Italy. Each driver listed on the rental form will require the compulsory International Driver's Permit, which is used in conjunction with your home D.L. These are easily obtained at any AAA office for a small fee and are valid for one year.

You'll also have to be extremely vigilant to avoid the dreaded ZTL (limited traffic) areas, as each pass through one of these will result in expensive tickets. Florence is just about saturated with automated ZTL cameras, but these also exist in many other towns as well (not always with cameras).

is that 2 bags for each person of the "we" or just 2 bags between two people?

Your questions are pretty basic ones about Venice which makes me concerned that you're trying to do all this without the benefit of a good guidebook. I strongly encourage you to buy at least the Rick Steves Italy book, if not the Venice one. It's loaded with all sorts of helpful information, including answers to questions that you don't know you should be asking. It will be the smartest money you spend on your vacation.

Happy travels.

And you may try to look at http://europeforvisitors.com/venice/

take a look at a map of Venice, and the answer to your question should be clear.