I have a rather big Knee type mill, that I would like to retrofit with either steppers or servoes, servoes would give the benefit of higher speeds, but how large should the motors be on either of the two.

The mill is a beaver NC5.. I'm going to change all the electronics on it for new, and lay out all new wires all around.

Alternatively if someone knows of anything from Ebay that would be usefull.. I have seen several servo drives, and servoes for sale there.. But I don't know what I should put together when I buy, I have been told that not all industrial servoes can be used with other manufactures drives..