Nice site!
I'm in the middle of doing the same project, but I already purchased the hydro motor a few years back.... just haven't finished it.
My motor was limited to 520rpms max (to simulate a 540 PTO). Granted, my tractor (Deere 2640, closed center, 2400PSI, 10GPM) hydro system is quite big and there's plenty of power to run it. It's funny that the manufactuers don't list the motor they use in hydro augers.... Any how, the sales man who helped said that it should do.....
Good luck on your poject. Oh, to connect the motor to the auger, you might have to fabricate a connector. I suggest getting a hy. motor w/ a 1" or larger shaft (keyed). Odds are, the input of your auger is a regular 540 RPM pto shaft (male). Hub City might have the right adapters (that's what I bought) that could be welded together to mount the motor to the gear box.