If you have space could use a lower speed, higher torque motor with a belt & pulleys?
I made an engraving spindle mounting for my milling machine which takes belt drive from the spindle with a 4.8:1 drive ratio giving me a 28 800rpm engraving spindle from the 6000rpm mill spindle.
You can buy round section Poly Belt in a range of sizes and it's fairly easy to cut & weld to size and pulleys are easy to make up if you have a lathe. My first belt has done 12 hours run time at full speed without problems,
Regards,
Nick