SD,
I try to answer at least some of your questions, in the same order as they were listed.
I never make reservations on line, so can't offer any comments there.
If you travel via the Brenner Pass, the train will be Italian at the beginning, but will stop at the border for a short time to change to an Austrian locomotive.
First class cars are somewhat more comfortable but I don't usually like paying the extra cost. As Rick says, all trains have both first & second class cars, traveling at precisely the same speed. You might like to have a look at www.seat61.com/Italy-trains.htm#What%20are%20Italian%20trains%20like
The location to store your bags will depend on whether the car has open seating or compartments. With open seating, there's usually a rack at one end of the car. This is often close to a door, so it's a good idea to at least clip the luggage to the rack in order to discourage someone from grabbing it quickly at a station. If in a compartment and it isn't full, many people leave large bags on the floor.
The bahn.de website indicates time of ~9H:10M on that route, with part of the trip by Bus. The time is shorter on the night train (~7H:39M), but you'd miss the great scenery through the Brenner Pass.
Rather than deal with the Bus and a long rail journey, I'd probably stop in Verona for one night, as travel time from Venice is only 1H:10M (earliest train appears to depart @16:20). The following day, travel from Verona to Munich (there's a train departing at 09:05, arriving 14:25 (5H:20M, no changes, reservations compulsory). Of course, these times are subject to change.
With this method, you should have no problems buying tickets from Venice to Verona a few hours before the train leaves. At the same time, you could also buy tickets for the Verona to Munich trip on the following day (which will include the compulsory reservations).
Hope this helps. Happy travels!
