Module has all your VIN and other information in it. If the used one was in a crash it does not work till wiped and all that put back in. If used but not in a crash then it will work but not having your VIN and other in there may screw you good, in a court case the insurance company lawyers can void payment claiming the system was tampered with. They fight over those modules in court and expect your correct info to come up when they read them.
Best is of course reprogrammed module to your car.
You MUST use at least same year for year module, no year jumping. And strongly suggest using exact same model of Focus (SE, SVT, etc.) so that you get best result. They make changes every thirty seconds in those systems.