I think you may be meaning the stepper drives rather than the steppers themselves?
If so then if the ones you have now work then I would just stick with them for the time being, cyou can always swap out for newer ones at a later date.
If by Lathe software you mean CAD/CAM then there is really not too much about that is free or very cheap.
Fairly cheap is Dolphin which I used to use and I liked a lot, it has a few annoyances but produces good code once you get into the swing of it, the CAD side however I did not like at all, so I never used that side of it.
I now use BobCAD and V27 has really come of age, once you get to grips with it it is very fast and produces excellent code/toolpaths. The CAD side again I am not to keen on, it is very powerful but a bit clunky and slow to use, especially when you have been used to the SolidWorks/Cubify/Alibre way of doing things.
Dolphin is probably the cheaper of the two but I think BobCAD can be got for just a wee bit more.
I have recently signed up for the Beta testing of OnShape CAD and I am really liking it, it is cloud based so you need to be connected to use it but it is very similar to Solidworks. It is well worth a look as it has a free version which is the same as the paid for versions but you are limited to only 5 open files at one time, for most people just using it for designing models for their Lathes/Mills, that would not be a problem.
Hood