Many thanks for the comments. I'm still building the frame for my new machine, and haven't reached the stage where I have to worry about single or double motors. It brings up a bunch of related issues, so I'll stop hijacking this thread and start another one.

One last vaguely relevant-to-the-thread point, though - I started out a couple of years ago with my MDF JGRO machine in order to make an engraved presentation plaque (immediate need) and the thought that I would be able to cut things like wing ribs, fuselage sides, etc. Haven't done any of the latter yet, but found lots of other uses for the machine that push the (admittedly very modest) limits of what I have. Spent the last couple of days profile-cutting 6mm and 9mm ply components for a slot-together architectural model, and watching the wheels go rounds at a mind-blowing max cutting speed of 700mm/min was tedious. The moral of the story is that if you are happy to cut modest amounts of material at slow speeds, you can get away with a real junk machine. But if you want to be able to do a good job at proper speeds, do it properly first time!