The hobbyCNC 3 axis kit with 305 motors works well on many systems of similar size. I built Joes 2006 and these motors drive it quite well.
I regularly cut 100 ipm on my machine.
There are other kits in the same range that would probably also work well.
Here are a couple of formulas:
If you know the load and want to calculate motor torque needed.
Screw Torque: in-oz = (8 * Lbs) / (pi * TPI)
If you know what motors you have and want force they can create on screws.
Lbs = (pi * TPI * in-oz) / 8
The efficiency of the system will take a little away from these numbers so don't cut it too close. Also motor output is not the same at all rpm, published numbers are usually max. You need the torque curve to get all speeds.
Go to the Geckodrive site and download the steppers 101 pdf under support.