I know of one very M-B knowledgeable individual who recommends that the vacuum modulator valve on top of a diesel's injection pump be replaced around 120,000 miles. I have replaced the one on my '83 300CD-T (at approximately 220,000 miles) and did so after following the testing procedure for this component as documented in the 617.952 shop engine manual. I consider it an expensive part at approximately US $120.00.
As far as the EGR valve is concerned, the one on my '83 did eventually fail (at about 200,000 miles) and I did replace it. Very expensive, around US $400.00. Like the vacuum modulator valve, I think it is best to diagnose the component before replacing to avoid unnecessary expense and work.