I just upgraded my wagon with first generation w126 rotors and calipers. I used a 1985 300SD to search for parts.
The only difference between w123 and 1st gen. w126 brakes is the front rotors are vented and thicker on the w126 and I think the caliper has a shorter piston. The rear calipers and rotors are exactly the same.
This upgrade is only for the benefit of the vented rotors and no different than a caliper and rotor R&R. I replaced the hoses and my front wheel bearings at the same time. There is no additional stopping power ... just more time to use it.
I used this write-up to help me. Thanks rhodes2010, it was helpful
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How To Replace Front Rotor W123
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