Deciding which parts to replace as part of PM is always the tough part of keeping up a used car. The decision is a function of the risk of any particular part failing at your current mileage coupled with where you are when it fails. Around town and close to home you're OK as long as the failure doesn't kill someone. A road trip in (for example) northern Canada is another matter.

The starter on my '96 E300D (W210, OM606NA) started acting up at about 270,000 miles. I bought a Bosch rebuilt on-line and replaced it. Starters usually give some kind of warning before they fail.

As to the alternator, the brushes (part of the regulator) are usually the first part to go. I advise buying a new regulator (about $50) and installing it, then put the old one in a corner of the trunk as a spare. This (a) puts a new regulator and brushes in the alternator, (b) gives you a known good spare to keep in the car, and (c) provides experience in regulator replacement that you can use, even on the road, should the new part fail.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970