I agree with John T, HP is at rated RPM.
A VFD will increase current to maintain torque within reason, but will not over current the motor.
I would get an 1800 RPM motor and build a mechanical reduction. That configuration will amplify the torque as it reduces the RPM.
I would expect if the unit had 3HP motor at 600 RPM, you would need at least a 7.5HP motor to accomplish the same torque at reduced speed. You could probably get by with a 1HP motor if using mechanical reduction.