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I'm having difficulty with reading your post, so I'll wing it...
The O2 sensors are not a "periodic" replacement item as it might have been on your old Audi. My own 1998 C230 has never had an O2 sensor replacement, and will not until they are required. The emission and FI systems in your 1984 Audi and your Mercedes have little in common as far as diagnostics are concerned.
As to idle quality, a failing Air Mass Meter often gives no idle quality problems and when it's really on it's way out will usually give a hesitation above 4500rpm or so.
It sounds like you prefer to keep throwing parts at it until the codes are cleared. Good luck.
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