The sanwa buttons do not require an extra ground, if that is what you heard.
If I remember right I think the Joystick and buttons were each attached to a separate connector to the pcb, with different grounds. But you're leaving the joystick alone, so you don't have to worry about it.
If you are fine with the dream, you will have no problems with wiring. I wouldn't pay someone to do this, since once you have the buttons in, wiring is dead simple. You connect the wires from the connector to each of the buttons, then daisy chain the wires from the ground to all the buttons. Then you're done.