well a 1/2 dia hole in some tool steel should take about 1/2 hp to drill and you should be able to run at 300-350 rpm. now depending on your vfd you may or may not have full torque at the low rpm of the frequency range and you'll have to check that. ideally your motor should run at 50/60hz at 500 rpm. when I was doing my calculations for the hp, it made more sense to use a 1500 rpm motor and over speed it vs a 3400 rpm motor and try and run it slowly, due to the torque penalty. but with a belt drive I don't think you'll have any problems.