Further to last: The Chinese servo controllers (only ones I know) have 5V optical-isolated inputs for the step/dir control, with separately excited inputs for the other discrete inputs (so you can use 12/24 for e-stops, etc). But, and this is the big but - the spindle speed inputs are 5V and can be directly coupled to a BoB output.

Edit: I liked the performance so much I replaced the phase/vfd on my ML7 lathe with a 2kW servo and drove that in exactly the same way.