"Actually I got no big problems except for sluggish performance, slow acceleration, hard to reach 100mph, high oil consumption. So I thought an engine overhaul might help, but apparently I am wrong."

Sort of like getting a heart transplant, when the problem was the flu. The first step is to take the car to a specialist, MB mechanic, and pay them to diagnose the causes of the actual problem(s). This is by far the most cost effective approach.

I know that when my '92 300E had a major out town break down at around 180K, the local MB shop first did the diagnosis, then I approved the repairs. It involved a leaking EHA, bad coil, suspect MAS relay. Totalled about $2400.00, which is what? 4 car payments.
Car has run like a champ ever since and is now at 242K.

Steve
'97 S320 116K
'92 300E Sportline 242K retired but running perfectly