Do you have access to the records to see if any engine work was done in the cars infancy?
Some of these cars had engine work done at 30-40K
(shortblock)miles
If thats the case you should be fine

but then agian the car may run for a million miles..some people never had any issues....I always wonder how that is

but seems to me if you change out to the 3.0 you will experience a loss in power...the last 300SDL I drove definitely had less power than my car and was pulling less weight
Also I think 3.5 liter is a quieter engine also

the MB zone rep told me some years ago it was a fuel issue...although I am not sure where the "bad" fuel was
One thing is for sure if the car is that old or has over about 75K miles expect no help from MB


good luck


Warren
1992 300SD 168K
Columbus Ohio