Getting the Mesa you want in the UK will probably be tricky or end up with a heafty import charge.
See how you get on with Linux, you want to be really sure it's for you before committing. I'd want the 7I80HD I believe.

I must sdmit, I have not tried using Mach3 turn with my UC300eth. I use the ucbb with mine on ports 1&2 for all the axis signalling.
Now, for the input ports however. I use these for signalling:
https://m.aliexpress.com/item/4000386156984.html?
My controls use 24v for relays, sensors, servo, etc.
A 24vin>5vout, 24 channel, npn, plugged in to the inputs on ports 4&5.
You can also opt for a 5vin>24vout 8 channel version to use on the outputs of ports 4&5 if wanted.
There are plenty of different types of these plc boards around fairly cheap.
I'm saving port3 for a high speed version later on.