there could be any number of thing wrong with it, you wont know until you get it running, which is why buying cars that dont run is so much cheaper! once the IP is fixed, there might be something else wrong with it, who knows. to be safe, i would assume that it needs $4000 worth of work and then decide if the car is worth $5500 to me in perfect shape. others might have a different number than that, its really just a guess.

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1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350