You can't get a correct answer of the proper RPM's to extract honey because RPM's are only one factor in the equation. What you are looking for is surface feet per minute. But to be honest, I've never heard of the correct footage you need to be running.
SFPM is calculated by pi times the diameter in feet, multiplied by the RPM's. The only thing is, the surface footage will be different for the top bar and the bottom bar of the frame, since they are at different diameters.
But I digress.
In regards to the homemade extractor, the fact that honey is acidic seems to have been overlooked. If you are going to have a motor inside the extractor, it needs to be completely sealed. Otherwise, the honey will burn it up pretty quick. Heck, you are lucky if a radio in a honey house lasts a year, because the tiny honey droplets in the air will eat it up.
If it is a food grade machine you will have the FDA involved. That by itself will double or triple the price.
Since when has the FDA had any oversight into equipment?
Here, even the local health department has zero authority in honey packing facilities or equipment. (And only large packers are required to be inspected.) It's covered by the Department of Agriculture.