There are two things that conspire to turn 123s and older models into rust buckets.

The first is the fact that the car is made with plain steel that has not been galvanized. The technology for continuous hot dip galvanizing of steel wide enough for car body use did not come about until the mid 80s. 124s and later are made with galvanized steel which is why you don't see so many rust bombs that are 124s or second gen 126s.

The second thing is that since the untreated steel will quickly rust, it has to be protected with something, either paint or another coating. The undercoating and sound deadening protects the steel when it leaves the factory. However it traps moisture over time, and if there is any kind of breach of the coating it is all over. Throw in road salt and it is a sure death.

I think the 123 is a beautiful timeless car but its fatal flaw is the steel. Unless you can find one from a dry climate that has been meticulously kept, it is likely going to have rust issues.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech