I don't know about "best" but my Super 7 conversion uses UC100 motion controller with USB connection to mini PC running Win10. A fairly generic BoB and 2M542 (or better DM542) drivers running on 40v. Unless you are going to fit new steppers or stepper/servos this would be fine I think. I can't recall if Mach4 uses just a spindle index pulse for threading or can take an encoder pulse too - I suspect the UC100 may not handle an encoder. But Mach3 threads OK with just an index.

Btw the Conect 121 is based on the Myford ML10.