You should have no trouble buying the tickets in venice. There are at least 2 ticket offices at a travel agency in central venice.

An IC takes about 10% more time than the ES. Sometimes the IC is a lot less than the ES. First class in the IC is a delight. it is not reserved.You much stamp your ticket when you get on. the ES requires a reservation. no stamp ticket.

If you have heavy luggage with rollers, follow the handicapped entrance on the right hand side of the station as you approach to avoid the steps. you can also look around for a loose luggage cart. I always leave them at the water's edge when I board a taxi(to the far right).

Take a through train. changing trains at bologna requires steps.