First off I should say I am no expert on this but am just going from my own experience of fitting out my lathe and mill.
If you keep the gearing the same then it would be safe to assume that an AC Servo of roughly the same torque would do you. Putting ballscrews on will reduce the friction so make things easier to turn but it will also increase the ability of the the cutting forces to be transmitted back to the motor.
The motors that are on at the moment should be approx 7Nm torque so you are probably looking about 1.5 to 2Kw AC Servo to get similar continuous torque. AC Servos have a lot more torque in reserve (usually 3 or more times continuous) but this can only be used for a short period of time, how long will depend on the drives/motors and lots of other factors such as temperature etc so it is always safest to base your calculations on the continuous ratings.
You are probably looking at around $1500 plus per axis for the Chinese motors and drives in the 2KW range, the drives I use are Allen Bradley and for a 2KW drive it would be around £1000 ($1500) and then the motor about the same.
Because these motors will have high resolution encoders you will need to use an external motion controller to get the speed required, I use the SmoothStepper and it works well for me.
DC Servos may be something else to look at but I have no experience with them. Simpson36 has done a review of DC servo drives and it looks like the Dugong and Whale are about the best, you can see info here
http://www.machsupport.com/forum/index.php/topic,12233.0.htmlHood