It could be the AFM, pull the codes to find out, if it's electrical it will show up there. If it's idling rough also then most likely it's not the O2 sensor, the sensor doesn't do squat when the car is idling and running WOT (wide open throttle). Most likely the usual tune up stuff that most people forget to do when they sell the car, plugs, wires, fuel filter, air filter, that stuff. What is an attarco engine by the way, some engine dealing/rebuilding company I assume? Also the MKIII runs pig rich naturally, probably not as rich as you describe but definetly more than most.