Dyna mechtronics DM3000 lathe , mach conversion

[Jonathan]

not sure where to start

I think start by giving it a thorough clean, then oil it. It looks like you've got it outside at the moment so you definitely want to put oil on the bed and get it indoors asap to stop it rusting.

what needs to go

It depends if you need to convert it to mach3 or not. If the existing system still works, then you'd save a lot of effort just leaving it as it is.

what to keep particularly with the tool changer:

Easiest option is just remove everything from the control box and start again, but you might be able to save a bit by re-using some parts. For instance the power supplies might be suitable for modern stepper drivers, so check their voltage and current ratings if you can. Look at what the spindle (which oddly enough I can't see in the photos?) is connected to - there's likely an encoder and some sort of driver for the motor. Also see what the toolchanger connected to.

From your pictures it looks like the stepper motors are Nema34. Have you tried googling the part numbers on them to see if you can find a datasheet? If you're lucky the current ratings might be written on them.