I am using AC servos for driving the spindle on two lathes and a mill. All are timing belt driven. I am VERY happy with them, and if money is not an issue I would use AC servo on any machine I shall ever do. I can do rigid tapping on all the machines, and some other things. Check out this video, where I am cutting a hex on my wee lathe:http://youtu.be/DajrTlI8IWA
Also, I should note that as far as I know the CSMIO supports rigid tapping with any spindle motor provided it has an encoder.
Delta servos are excellent. Properly tuned you should not experience resonance. However, if you belt is too long you may need to use some tension wheels along the way. AC servos have full torque starting from zero rpm. You can run it in the rpm range you ask. Your original motor 1400W @5000rpm provides a nominal torque of about 2.6Nm. If this satisfies your needs, then this is the torque I would go for when picking a new motor.