If you want a 500 size motor, the Graupner 500E is a 12 volt motor which will give about 12000RPM at 12 volts. This will need gearing down, to around 2000-3000RPM to drive the propulsor, which means reduction somewhere in the region of 4 or 5:1.
MFA/Como do a nice 12 volt 500 size motor with planetary gearbox at 5:1 already fitted, which is worth investigating-
http://mfacomodrills.com/gearboxes/942d_series.htmlWhether you go for the lower or upper end of that RPM range is dependant upon the performance you are after (scale speed will be about 3.5knots). The Vanguard is a long streamlined sled of a boat, so quite slippery. The propulsor looks very cool, but doesn't appear to be as efficient as a conventional prop in model form (too many blades I think). To further complicate matters, the propulsor you will get is different to the one I have. Dave Merriman made new masters for the propulsor (along with the sail, rudder and hydrovanes) going from a 15 blade design to an 18 blade design with a different blade shape.
Both the earlier prop and the later prop were tried out in Jim's Vanguard at the Summer Dive-in, the later 18 blade prop proved to give better thrust.
However I have carefully reworked my propulsor by re profiling the blades of my prop to make them thinner, sharper, and give them more curve.