Oh yes... ancient indeed - the newest one is 20 years old!

Well, I will know more when I look at the 420SEL on Sunday. If the timing chain/guides/rails have not been changed (written receipts/verification), I am going to offer $500 less so I can have those replaced ASAP and not have that guessing game like I had with my 380SE. Since the 300SEL has the least number of miles and appears the nicest it may be a good choice.
The seller of the 300SD has other Mercedes for sale and all seem to have a very similiar description in the write-ups, BUT they are all very nice cars and seems to have a good rep.. so there.
Thanks for the tips/replies.