I have sold quite a few servo systems and servo motors for full size mill/drills.
I have my own controls and motors mounted on my mill drill. to answer your question I need some of the parameters. Are you planning to use the existing acme lead screw or are you planning to install ballscrews? It make a big difference. The existing acme leadscrews probably have .010 backlash in them and they are only 45% effricient. A ballscrew is 90%+ efficient. Thus you could have twice the speed with ballscrews and more power to the axis.
Then the next question is do you plan to direct drive it? Or do you intend to use a pulley ratio?
What rapid traverse speed are you looking for?
I have sold quite a few servo systems that use my 400 oz in servo motors with a 4/1 reduction that gives good speed and plenty of power. I have also sold quite a few systems with my Ametek 600 oz in servo motors witha 3/1 ratio timing belt drive and last I have sold several systems using my 800 oz in peak servo motors direct drive. You can contact me off line at firstname.lastname@example.org for more information. Since I retrorfitted my own mill/drill I can certainly give you some pointers.