Hi,
menardmfg is quite correct, if you want more inputs for buttons and switches then the PLC such as he has mentioned would work well and at a good price.

Any signals that are received by the PLC that need to be communicated back to Mach will have to pass through the Modbus serial link with a latency of
tens of milliseconds. That would be fine for a button to turn on the dust extractor fan say but if you wanted say an index signal back to Mach then the
latency will ruin the timing. Likewise for a probe signal your motors need to stop within microseconds (ideally) of a probe contact. So...probe contact event,
signals Mach 10ms later which signals the motion controller to stop the motors say another 1 ms later.....too late the probe has buried itself in your work
because the motors did not stop in time.

PLCs themselves are usually very fast, so it can detect an input event and very quickly control some output, within  tens of microseconds, usually more
than good enough for automation purposes. The trick here is that it can control its OWN outputs quickly but if it has to signal another device, like Mach,
THAT can be slow.

You need to decide if the extra inputs you want are going to be time critical. Two that I have mentioned are probe inputs and index signals for threading.
If those are part of your application or you may want them in the future then an ESS is a good way to go. If however you have no time critical inputs
the a PLC is good. The DL05 that menardmfg mentioned is good value and can be extended with analogue modules at very reasonable cost.

Craig