Which machine is it you have?
I think alot of people are on vacation but I will try and help you as much as I can.
This sounds like I older mill and I expect the parts ( table and sadle) may be heavier than you expect. A horizonal mill can get into some pretty heavy cutting also and if you are going to be cutting steel you may need some pretty good motors.
A screw with .750 is pretty odd. Are these ball screws, or lead screws?
Just a first thought it sounds like you might need a good servo setup. But steppers would work also. DO you have any thoughts of which one you prefer? If I am thinking right you should be able to use either one but with the speed your looking for the servo's might be best.
Does this machine have real nice axis movement? In other words does the table slide very nice? Or is it pretty stiff? If its a smooth axis with ballscrews you might be able to use the 350 oz/in servos with some geckos which would make a very good balanced package. If yo will be using lead screws then more power will be needed.
SO a little more info on your setup would help to get down to what you need. Also budget, and what other machines you might have to work with. You will have to make motor mounts, and buy or build bearing supports.