Assuming you’re making your own rotor, that’s likely the best option at this point, yes.
We used to sell the M100/M200 as standalone motors that weren’t built in to a thruster housing. We’re currently working on bringing back the M200 with some extra robustness, but it likely won’t be available again until some time next year (we have several ongoing development projects).
Not that I’m aware of. From a quick forum search of “T200 gear” / “M200 gear” the following comment seemed the most relevant, but you may find other interesting tidbits in other comments/threads
What are the main differences beteen the T and M that might cause me problems in adapting a T200 ROV core and rotor to attach to a gear train? Is it just the stator mount? Would I have to attach the gear train to the post or the barrel of the rotor (or either)?
As I understand it the primary difference is the stator mount - the arms on the T200 Core aren’t non-destructively removable, and it doesn’t include a flat rear with holes that are intended for mounting to. The M200 Usage Guide is still available, if you want to find out a bit more about it
The T200/M200 Rotor is a shared part, and I don’t expect that would change.
How you mount things is up to you. The simplest options I can think of would be
the two M3 propeller mount holes on the rotor face, or