Your looking at this wrong way around. The power/torque curve at 20k rpm will be lot lower than at 60K so your more likely to snap cutters from spindle bogging down.
Regards cutters snapping from vibrations then it's often a weak machine frame it's self that causes vibrations rather than just the spindle. Only on very tiny cutters, sub 1mm, will the spindle be to blame and even then it will be mixture of both.

If you want to oversize then spec it so the power curve suits your cutting conditions. IE: if you want to cut at 20K then get spindle with the highest torque is at 20K.
Often that is roughly 2/3rds the rated RPM so 30K would suit.

However, I can tell you without a doubt, because I've fitted in excess of 100,. that Chinese 2.2kw 24K Rpm spindle with do what you want standing on its head 12hrs per day 7 days a week.