Hey man, I have done several of the conversions. You mentioned steppers for a large mill like that look for some very beefie steppers. Look in the 800+ range. You will want to work the beds as well. A little elbow grease and some good compunds on the ways and beds since you have it apart. The other thing, Ball Screws! Check and see what is being used. Duplicate them in a good Ball Screw with dual nut 0-backlash set up. If it does not have a lub system, when you have it apart would be a good time to add one, they are simple to make up.
Why just 2 axis. You will just do the other axis soon anyway! I mentioned the larger motors, make sure you look for boards that handle around 10 amps or more. Also you will need a beefer power supply for the larger motors. They are out there. You can Ebay most of it. Ball Screws can be done for around 300 bucks.
Good Luck, if you need any help just let me know.