An RC motor was the original plan for me I couldn’t find gears/belts very cheap one set cost about as much as a 660 in the US. In the beginning we used some other tools, and settled on the 660 in the US because of it’s price. The original flex shaft spindle was pretty good, so small and light, but it was expensive. Cost is a rough one, pretty different over the globe.
Keep us in the loop on how it goes. I’d love to see what you have in mind for your closed loop. This alone would make most machines preform so much better regardless of spindle chooice.
As for feeds and speeds there are tons of calculators on the web, the numbers are for different kinds of machines but will give you an idea of what parameter changes what.