It's very hard to find a used car that doesn't need something expensive now. It's therefore almost impossible to find a used car with 300,000 miles that won't need something expensive in the next few (1,000? 50,000?) miles. Even a Mercedes wears out and they are definitely expensive to repair -- especially the OM603 engine in your '87 300D. With fewer miles (150,000--200,000) it would be much less risky. If it was a case of using it only to run around town, sure, we'd say "go for it" (depending on the price, of course). However, as a long-distance car for the family, there are too many expensive things getting ready to wear out (timing chain, IP, transmission, etc.) and all the time the main and rod bearings and the rings are getting weaker and weaker. Since you can still find nice examples with a lot fewer miles, I'd suggest waiting.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .343,000 miles Wife's car
2005 E320 CDI . . 148,000 miles My car
Santa Rosa population 177,300 (2026 projected)
Total. . . . . . . . . . . . 668,300
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970