Replacing Servo motors

[i2i]

They're running at half step on a 2.5:1 reduction on 5mm pitch ballscrews. Current is 3amp.

[Jonathan]

If you can afford servo's from DDM they would be excellent, since you'll get high rapid feedrates and much higher resolution:

http://www.dmm-tech.com/Files/DHT_Servo_Motor.pdf

Their 60EM-DHT-36, 400W servos would be plenty, or even the 300W servos. Send them an email and they will calculate which is best suited to your machine if you know the mass each axis is moving. I've not bought from them yet, so can't comment on the quality first hand, but from reviews I've seen on CNCzone they seem to be excellent...but it will cost more than a Nema 34 stepper motor system.

Or there's some on eBay, but you said you can't afford servo's so...

If using Nema 34 stepper motors I would get mains voltage drivers since this will greatly increase the torque at high speed. For example:
Zapp Automation Ltd - Stepper motors & Stepper Motor Drivers - Servo motors and Servo Motor Drivers - Ballscrews & linear motion products. : 2M2280N (High Voltage Stepper Driver) [2M2280N] - £176.40
Although they've gone up in price since I last looked, so maybe:
CNC Cut Nema 34/42 stepper motor driver DQ2722M,7A 110-220VAC www.wantmotor.com | eBay
or
2MA2278 CNC Stepper Driver For Nema34,Nema42,Nema51 Stepper Motors 7.8A 110/220V | eBay

If I actually had something useful to do with my micro lathe I'd get one of those drivers for the spindle.