Off the cuff, I would say to avoid using stepper motors on large machinery, unless you can find a closed loop controller system that will detect and correct for lost steps. I do not think Mach will do this yet.
Ajax cnc or Camsoft cnc, that is a couple of the more often mentioned names around here.
The level of precision required for a useful lathe X axis is very high. This requires lots of steps, and to obtain decent rapid rates, you need a controller that can crunch lots of numbers, fast. Camsoft, and the Galil card that it often uses, can count as many as 12 million encoder counts per second.
Ajax can likely give you some advice over the phone about what you've got already that may work, or a complete system.
Camsoft, well there you'd be able to use a bit more of your programming smarts to write some logic. It takes lots of time, but, it is a versatile system. If you are going to do a mill and a lathe, you might get double use out of a Camsoft system and the time required to get it running. As a bit of a tinkerer hobbyist, you might enjoy the battles to make the system work as you envision.
Sorry, but I know of no really cheap way to do a professional quality retro, and on a larger lathe, you don't want any whoopsies coming out of the woodwork that did not originate in your g code.