Decision is now leaning towards throwing the old motor back on!
Why?
One reason was that i don't have any idea if a 1.8kw servo will be a good replacement and 1.8kw seems to be the only (affordable) size that runs to 3000rpm, also, fitting an encoder for the rigid tapping module would be a total ball-ache on a Bridgeport - there is no clear access to the spindle shaft and I wrongly thought it would use the motor encoder signals which it does not.

So, back to plan 1 until the bearings fail 100% is to use the standard motor with my VFD and use the variable mechanical speed to give me two electrically variable speed ranges that overlap.