Thanks again for the comments. My problem is that I understand the theory of the control systems and the various bits that go to make it up, but I have no experience with the actual hardware, even to the level of knowing how interchangeable things like encoders are. Is there some kind of mechanical standard (like NEMA motor casings?) or would this mean adapter plates, spindle couplers, or whatever? This might sound trivial to those who have done this kind of thing but it's a bit of a barrier to entry for someone like me!

I could leave the servo motors in place, but almost certainly have to replace the encoders. Single-ended, differential - yes, I can cope with that, especially if there are off-the-shelf converters available.

I could leave the existing servo drivers in place (yes, Dean - I'm pretty sure from the part numbers that these are rebadged Glentek) but then need an analogue motion controller, I believe. As an alternative, are there modern DC brushed motor servo drivers that would take step/dir input? That widens the motion controller choice.

At the motion controller level, I could use either a self-contained system, complete with display, etc (like the SZGH or Newker or whatever), or a PC with motion controller (PV ET7? Probably not CSMIO-IP/S or A because that means Mach3/4 or the CS Labs equivalent, none of which seem especially desirable), or halfway house with, say, ET7 plus dedicated single-board Linux and display. Are there other likely candidates that I should look at?

It's really the driver stage onwards that gives me the most uncertainty purely due to lack of experience in replacing driver/encoder/motor "stuff". I've been a stepper and EM806 kind of guy to date...

My mate has been doing some research of his own but given the fact that he understands even less than I do, we're going to have an interesting conversation soon!