The only part of the project I understand is the USB keyboard/barcode scanner pass through.
A Leonardo with a USB host shield should work as a USB keyboard pass through. Take a look at the following.
https://forum.arduino.cc/index.php?topic=334626.msg3554328#msg3554328
If the EPOS system does not recognize the Leonardo as a keyboard more work is needed.
https://forum.arduino.cc/index.php?topic=545288.msg3717028#msg3717028