The biggest nightmare that I am aware of on the W123 turbo diesels is the R&Ring the starter. It is very hard to get in and out. Also check to see if the cruise control works, they are notorious for failing or working poorly. Another thing, is the car a CA model? Then you must make sure the the trap oxidizer has been replaced. There was a recall campaign from MB that replaced them for free. The price seems steep but if it is in pristine condition and has had many of the major components replaced then you might be ahead of the deal in the long run. You might save some frustration and won't have to put in a lot of extra money on a cheaper car.
Stay away from the W124. Very complex and expensive to fix. Not a DIY car.

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DJ


84 300D Turbodiesel 190K with 4 speed manual sold in 03/2012