Any plug-in Z-Wave will also act as a repeater, so I would get 2 dimmers or 2 switches (or even convert the closest outlets to Z-Wave). Plug one switch/dimmer into the shop outlet closest to the house and plug the other in the house closest to the shop.
"Z-Wave’s devices can communicate point-to-point for up to 120 feet on their own, but with their ability to hop signals, effective ranges of up to 600 feet are easily achieved, and Z-Wave networks can be linked together for even larger deployments. " - http://www.z-wave.com/faq