How is your friend's mechanical aptitude? Is he familiar with the CIS fueling system? The engine is the only real difference from the diesel cars of that era. The CIS system is remarkably reliable when maintained and performing adequately, but can cause even the most pious of mechanics to belt out a stream of swearing that would make sailors blush when something goes wrong.

If your friend isn't mechanically inclined and isn't willing to learn about the CIS system, walk away. If he has to have a shop perform the work on the engine, he better have deep pockets.

One of the best things you can do for a CIS car you want to turn into a daily driver is to replace *EVERY SINGLE ONE* of the rubber vacuum fittings under the hood. Vacuum leaks are the CIS system's biggest nemesis. With the age of the car (mileage doesn't matter), the rubber is likely hardened, alligatored, or softened, depending on where it is in the engine bay.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]