I am watching this thread with great interest because building my own spindle and interfacing it with mach is something that I also would like to do.

The mechanics will not be a problem for me but designing electronic boards and putting them together has never been one of my strong points!

What I do have though is a selection of various good quality DC motor speed controllers that may be of use and would be happy to send one to you if it will help. Let me know the spec of the motor and I will see if I have anything that will be any good to you.