The speed you'll be able to run the motor at is a function of the resolution of the encoder and the step rate that your controller (Mach3, EMC2 etc) is capable of outputting.
If you have a 500 line encoder on the motor it takes 2000 step pulses to turn one revolution. If your controller can output 45000 pulses / second your max speed would be 45000/2000*60=1320rpm.
If you then couple that that motor to a leadscrew thru a 2:1 belt reduction the top speed of the screw is 660rpm and if the screw has a pitch of 5mm you'll get 3300mm/min.
The smallest increment of movement (resolution) would be 5/2/2000=0.00125mm.
Want more speed? Increase the pulse frequency, reduce the encoder resolution (or use the step multiplier in the UHU), reduce the reduction ratio between motor and screw or increase the screw pitch. All of the above, except increasing the pulse frequency, has the drawback of also reducing the resolution of the machine.
I'd say most machines today uses brushless motors (AC servos or brushless DC) for both axis and spindles. Brushed DC motors are "old technology" but works just fine. What voltage and current ratings does your servo motors have?