Jim sez: "Problem is, they're not common so they won't go for free. They probably will command a premium even when bought new because they sell so few. Three phase motor, of, say, 7 hp, with some capacitors and a potential relay to start it?"
Yeah, Jim, that'd work. Or for the less sophisticated, some sort of a jury-rigged pony motor.
Jerry has the solution, though, when he says go for a surplus 3-phase motor and run it on single-phase. That theme will work right up there to where you can't get heavy enough 240 volt service to start and run such a beast. I'm looking for a single-phase 300 HP motor from a streetcar or diesel loco, myself. Of course, that would probably be a universal (commutated) motor; hey! while I'm at it, I could get a genset also.
Bob Swinney