For $3k I think you have a good deal (assuming the car is in as excellent shape as you imply). If the engine rebuild is questionable it may be closer to a 2k car or less.

Worst case scenario the engine bends a rod and starts consuming oil. You replace the engine with one from an 86-87 300 SDL. You may have to wait to find the right one but I've seen them lower than $1000 with a good engine and bad body. If you planned on keeping the car a LONG time (like the rest of your life), after you buy it I would be keeping my eyes open for a rusted 300SDL with a good engine to buy and park or just take the drivetrain out to keep and sell/junk the rest.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD