Retrofit help

You only need spindle encoders if you are planning to use spindle-synchronised motion. (lathe threading, for example)

If the drives use analogue control signals than the usual choice would be the 7i77 card, connected either to a PCI 5i25, PCIe 6i25 or an ethernet-connected 7i92.
Given that the 5i25 and i25 are out of stock, the 7i92T(M/F) looks like the likely choice (the M/F is which gender of DB25 is fitted. Check with Mesa which one you would want)

There are other options, for example the 50-pin boards (such as 5i24-25 + 7i36 as an in-stock option if you want to use a PCI card solution. (The PCI-e 6i24 is shown as out of stock) )

In both cases a 7i44 or 7i74 can be added to allow up to 8 smart-serial peripherals to be connected.