If you're looking for a used, parallel port KX3, then a look at the CNC4PC C11G parallel breakout board docs would be helpful. This is the BOB on (at least) the ArcEuro KX3s. If you want to replace it with a USB or ethernet board, in fact any other controller, bear in mind that the C11G has a 0-10v spindle-control output, controlled by a stepper signal, and you'd need to replicate that if you wanted software speed control.

If you want any more info about the KX3 I'd be happy to help.