Diagnosing control units is the worst of business, but I would take my car to a tech who stands by his diagnostics.
If I diagnose a control unit and I install it and it doesn't fix the car the part comes back off and becomes inventory. If I diagnose a control unit and you replace it and its not the problem it becomes your inventory. Depending upon risk the amount of testing changes.
For a car like yours I would monitor two outputs: EHA current and Idle speed valve current. if they were wrong and the basics to the controller were right I would make the call, I would get a used controller (actually I probably already would have one) and confirm the diagnosis. The customer would pay less than half the price of new.
Bosch Master, ASE Master, L1
33 years MB technician