You don't need to go to a 9 speed FDER & crank, although purists might be offended.
I've upgraded both my bikes to 9 speed and the existing cranks/FDER's worked fine.
For a short time, I was using an old cottered crank from a 2X5!
You did say that you needed a new crank, so I'd just get whatever MB crank in 8-9 speed that suited your fancy.
Pick out one of the online gear calculators and determine which cassette to use.
Just because you can do whatever on a trainer is pretty meaningless in the real world.
There's this thing called wind resistance that sucks up much of your energy at higher speeds.