I'm not sure I would rule out a 300SDL just because it could possibly have head issues. If you do that you are going to rule out alot of automobiles.
The thing to understand is that from '86 on all of them had aluminum head versus the cast iron head on the '85 and earlier models. Cast iron is ALOT more forgiving than the aluminum BUT on the same hand pretty much all new cars these days have aluminum heads. The problem arises when they are allowed to run hot or the coolant is not changed every year or so.
On the other hand though - if I purchase another MB it is going to be a 300SD
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