The venerable old Mach3 system works well on a lathe, I use it on my converted Super 7. It works for threading using a single slotted index on the spindle. Mach3 just uses this to tell it when to start each threading pass, it doesn't control spindle speed , just assumes there's enough power not to slow down. That works for me, I don't cut very deep threads. You have a 1/2 hp motor, could be fine, just try it and see. I believe Mach4 can use encoder pulses from the spindle for finer control, not sure about that. To reiterate his point about the controller, if you want to control spindle speed from the PC you need to make sure it's isolated. If you're happy with manual control there's not a problem. I have a Denford Novamill of the same vintage, it originally used a Sprint drive but I use a KBIC drive with manual speed control.

As m_c says, there's no point in using closed loop steppers, you might as well stick with the original motors, at least to start with. Upgrade if needed. They will probably work much better with modern microstep drivers.

Regarding the PC interface, I use the CNCDrive UC100 on both my lathe and mill, USB interface to a Win10 mini PC.