If I got the /30 do you think I could get away with keeping the chain?
I would always put a new chain on when changing cassette. Like I said yesterday, a worn chain will cause unwanted wear on the new cassette. That said, I do use the same chain on both my NEO and the rear wheel. They (and the chain) are all Ultegra however, but I don't think it matters. Optimally, I should probably switch the cassettes around every now and then to get equal wear on both.
If using the bike solely as an indoor bike, your chain/cassette will live long... Taking it outdoors and then the dirt and grime back onto the trainer - not so much.
Also be warned that you might need to adjust the gears when moving to/from the trainer, if using the bike with a rear wheel too! Or just live with it not running 100% smoothly - I know I do.
I know you might have to work another weekend to buy all the extras, but that's life!
