An ethernet controller is a better option than USB. You can get a cable to plug the G540 directly into the UC300ETH. But I think the UC300ETH will be overkill for your application. A UC400ETH would probably be a better choice.
The G540 has terminals for the additional input and output pins, so you don't need a breakout board. But if you want to use the additional ports on the UC300 or UC400, then you'd need a breakout board for those.
And while UCCNC is my preference, you can also run Mach3 or Mach4 with these controllers.