I haven't owned this specific car, but I am of German heritage and I believe I can speak for this era, that the reliability is TOTALLY about whether the owner has followed correct maintenance intervals for service. If all required service has been performed meticulously for the entire 37 year life of the vehicle, then it will remain reliable indefinitely. And if that maintenance has been inconsistent, you will be punished by unreliabililty. It is the German tradition that incorrect maintenance procedures will not be tolerated. Follow correct process or be disappointed. As such, be very careful who you buy from, and carefully review maintenance records.
Note that more modern German vehicles tolerate american variance in attention to maintenance better. Vehicles from the 70s would not be so tolerant though. Everything must be done in rigorously correct fashion exactly according to owner's manual maintenance procedures at all proper intervals. It is a legal contract. If you do not follow the owner's manual, it is as if you have given a divorce and the car will feel free to cheat on you.
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