In 2002 I paid $4500 for a 77 300d with 55k original miles, original owner always garaged.
This fall I paid $5900 for a 79 W116 300SD with 111k miles, second owner, spotless car with a new ACC servo.
$3k seems high to me. I'd rather pay a little more and get a lower mileage example. Servo replacements are running $400-1600 nowadays. With a dead compressor, it would be easy to drop a couple of grand into the climate control system.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13