I was planning on using it to make a motor for a milling machine, to cut metal and wood. So it’ll probably need a high torque and rpm. Do you have any suggestions on what I should use, and any tips for making the electromagnets? Thanks!

 

What is the minimum I can reduce these to?

According to the Instructables article, down to 4.7u has been tested. That doesn't mean you can't go lower .
For a milling machine application that circuit won't provide enough power, plus you'd have to have a speed control. Have you considered that?

 

I had a feeling it was gonna be either manufacturing or data storage. The precision position stepper motors you require are not DIY I'm afraid, they have other components than just the motor like indexers and drivers plus a user interface (PC). They are also precision manufactured to match shape with purpose (e.g. more steps, more teeth/coils). My advice is to either buy one for your machine and retro a LIMITED CNC capability from it or...... buy a CNC miller.

I'm assuming this is what you're after
EDIT there is a kit it seems
http://www.ebay.co.uk/itm/Germany-Ship-3axis-nema23-stepper-motor-425oz-in-2-ph-3A-dual-shaft-CNC-Kit/130998735763?_trksid=p2054897.c100204.m3164&_trkparms=aid=222007&algo=SIC.MBE&ao=1&asc=20140407115239&meid=53226af9485740198044c83be3d13b88&pid=100204&rk=20&rkt=30&sd=331395484365

Sorry I'm not sure if I can be of any more help on this topic