Lathe Retrofit.. Threading?

I'm planning to retrofit my Lathe (10x28) to CNC.. (or buy a mini-lathe to retrofit first..
haven't decided yet really). I've decided that a CNC lathe is more useful in the short term
that retrofitting my Mill/Drill right away (and thanks to the posters who answered
questions about that!)
Either way, I DEFINITELY want to be able to thread on it!

I"m not at all clear on how exactly to get it to pick up the same spot on sucessive passes
of thread cutting.

I've sort of gathered that some kind of encoder on the spindle is needed... but I don't
know what kind, and I don't know how to connect it to a controller (for example, the
Gecko stepper controllers).

What kind of encoder are we talking about, and is there a standard way to provide the
controller (or computer) with the feedback needed to cut threads?

Or should I retrofit my Mill-Drill first?

Thanks for any tips!
Paul F.