I agree that the diagnosis sounds wrong.
Having said that, if you have the original timing chain, tensioner, and guide rails in your '89 420 SEL, I don't think you'll find a person on this site that wouldn't recommend replacing them.
Do a search on 'timing chain' and you can read these threads all day and night.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".