Thanks everyone for the in depth information - lots to process here.
Another question - is there much point in doing the PADI Rebreather or Advanced Rebreather class before owning a rebreather, since the training is unit specific? (or any other agency, I know it's more about the instructor than the agency)
IMO, no. Your best bet if you don't have an idea which unit you want going into it is to do a couple try dives. Get a feel for different units. Try a unit with FMCL's, BMCL's, internal CL. See if you like the manufacturers electronics or the Shearwater controllers. Try an mCCR and see how PO2 acts.
Manufacturers and agencies have a strange relationship. Manufacturers have to approve instructors, even though it's teh agency material and c-card. Some instructors who teach for multiple agencies may be able to issue different certifications for the same class, or may be able to choose to issue one agency cert because it is more liberal or conservative than another agency's cert for that specific rebreather.
Alternatively, if you have an instructor in mind that you really like, it's entirely possible that the unit s/he teaches is the unit you'll want to do your MOD1 course on. Even if it's not the unit you're going to buy, you'll be able to train the material and skills with the instructor you learn best from, and then you can crossover to the unit you want to buy later on. Crossovers are easy, it's learning the in's and out's of the unit itself, whereas MOD1 teaches you how to dive a rebreather in general.