I have no idea if the big six flywheels bolt on to the baby six motors. The early e30 etas as well as all the e30 i's are all single mass which is the way I would lean.
You know this flywheel fits and is made for your motor and I'm sure its way more plentiful and cheaper than an m5 flywheel.
Another thing to consider that I can't answer is which starter would work with the flywheel you choose. I'm pretty sure your current starter will work with the e30 flywheel, but I don't know about the others.
A used flywheel is a safe buy, just make sure its in good condition... not too much scoring, not warped, all the ring teeth are there etc.
I believe the dealer installs an e30 clutch for something like $650 - if this is the case for your car you might be able to get everything done for about a grand.