I would select the ball screws first. For a machine this size, 25 - 30 mm dia ball screws would be suitable. A pitch of 5 to 10 mm would fine too. Install the ball screws on your x and z slides and then measure the torque that is needed to turn the screws. Servo motors are selected after you have the required torque and the desired max. rapid feed. For conventional slides that have metal to metal contact, I normally don't go above 5000 mm/min. So if you have 3000 rpm servo motors and a 5 mm pitch ball screw, the timing pulley reduction to get 5000 mm/min rapids would be 1:3. And if the calculated torque is 6 N-m then for a 1:3 gear reduction you'll need 2 N-m on the motor shaft. A 2 N-m, 3000 rpm servo motor would be roughly 750W. This is normally the size of motors on machines this big. The spindle encoder mach3 needs is single pulse per revolution. If you are going for the parallel port setup then make sure that your index signal is long enough for mach to read. With external motion controllers you normally don't have to worry about the index pulse width.