Unfortunately, I am a Jack of all trades and a master of none. So I could not answer most of these questions in any "great" detail. But how about this. Lets start with a problem like this since it is getting cold out:
Will not start....
If you guys give feedback and suggestions I will consolidate all the posts into a word doc and post it. If not I will lay the subject to rest.
A

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1987 300SDL
187,000 miles

Here's another scenario - there's a wonderful 50+ step AC compressor checklist graciously shared by someone whose name/handle I don't remember. That should be part of the 124/126 checklist.

Sixto
87 300D

http://www.peachparts.com/shopforum/members/saumil.html

Another brilliant fellow who is rarely seen...............

Another checklist opportunity is what to check when considering a used car. gsxr has a good 124 list floating around. Should be a sticky. I'm surprised there isn't a comprehensive list for 123s.

Sixto
87 300D