I would start by finding the torque that your spindle will produce. This will be the maximum cutting force that your machine will see. If you are running 12mm with a 5.6KW spindle @ 24000 RPM the cutting force at maximum cut is pretty strait forward. I am sure guys around here with more experience then I could correct if I am wrong but it makes sense as to the maximum load that the machine would put on the cutter. for example .5" 7hp spindle at 24000 RPM would have 1.53 ft/lbs of torque. That translates to 73.4lbs of cutting force on the cutter. how big of motor you put on the machine is a matter of engineering economics.