Universal Truth: all m180, m130 and m110 inline 6 cyl engines need valve and head work at about 125-150k miles. My last inline 6 was twincam 280SEL w116. It had 189k miles and ran like a bat out of hell. Previous owner did a valve job at 145k miles. Also owned an m110 280/8, did a valve job on it and replaced the timing chain at 125k...... Then drove the car up to 289,000 miles. The engine was running strong at that point but the body rusted to smitherines.

General rules on mandatory head work include replacing all guides and seals, burnt sodium filled exh valves cannot be reground but must be replaced, and remove as little metal as possible from the cyl head surface or better yet none at all unless it's warped. Intake valves rarely need replacement and can be gently lapped to reseat em. The entire operation should cost less then $500 in parts and machine shop costs, pulling the head yourself. Deffinitely a bargain and money well spent.

And never replace the rings on the MB inline 6 or you will end up with piston slap. Your compression test readings sound pretty good to me (except for #4). And how many miles did you say were on the car?