Yes the 3.0L six is a fine engine, smooth and more powerful than the old
five cylinder engine but not as tough as the old five. If you want a
300SDL buy an '86 or '87.
The 3.5L six is an engine to be avoided, IMHO. It's prone to bending its
connecting rods. When bent, the piston attached to a bent rod is no
longer square to its cylinder so the cylinder walls get scored and the
owner has an expensive mess to fix.
The 3.5L engines' durability varies greatly, there's no particular
mileage to these failures so one just doesn't know. Replacement
connecting rods have been modified and they seem to be OK.
These 3.5L 350SD and 300SD cars are often bought used by the
unsuspecting and have caused much unhappiness.