Based on several experiences over the last four years in replacing the tumblers in my four W123 cars (I do this now as a maintenace item as I have had tumbler failures in the past) the cost of a new genuine tumbler with a single key that fits all the locks on the car is approximately $170 (MBCA club price). The classic center does require proof of ownership. The entire ignition switch (diesel) without tumbler is actually cheaper at around $150. I have never heard of a dealer requiring the tumbler to be installed by a dealer technician or other certified mechanic.