Looking on the ticket site IC585 depts Mestre but arrives in Firenze Rifredi where is this station.
The thing is that "main" Florence station (SMN) is the dead-end station, while "Firenze Rifredi" is the previous one before SMN, it allows to stop in Florence, but at the same time not to lose time for the change of directions as with SMN.
Basically it's only 3km away from SMN, you can see the exact location
here. To get from Rifredi to the centre you can eaither use city transport or simply board next local train which goes to SMN (for instance next ones depart Rifredi at 12:27, then 12:44, journey takes only 5 min.)
We were looking at this train because we are not in a hurry and was half the fare (mini 1st class).
Smart choice, the tickets itself almost twice cheaper then the ones for EuroStar, but at the same time it is slower only for about an hour, not a crucial shortcoming in this case
Also when booking the tickets is there any way to determine or request an exact seat eg facing the way the train is travelling.
Well, that is a bit tricky, since there is no definite answer which type of carriage you'll have, it might be either compartments or open airplane-type salon, plus numbering might differ even in the same type carriages.