I too have benifited from your proposed senario a couple of times. Usually I inspect the whole car to help make a decision. Depends what else shows up in the process like too much deffered maintenance. For example the last owners just drove it and fixed nothing as long as the car was mobile. Or everything else seemed good.

The car seems a little too cheap if in basically good condition. On the otherhand the individuals circumstances may be playing a major part as well. It is very hard to say without seeing and inspecting the car personally.

A buyer can wear rose colored glasses for example. A neutral party with some knowledge usually does not. In your case quite a lot depends on your experience and ability to judge the seller and car accuratly. Could be anything from one hell of a deal downward.

Unfortunatly at arms length on this site an absolute guidance cannot be given. We cannot personally inspect the car for you really. If the car is in basically good shape otherwise by my standards I would buy it.

At his price if the car is basically good it may be expecting a little too much for him to tell you why it will not run. There is also a good chance he does not know. That may be why the price is what it is.

Fair to say everything is not always clear or easy? I wonder if he might let a mechanic of your choice have a look before purchase? Just do not repair it or get it running before buying it. Diagnostic by a good mechanic familuar with this model should be enough. Pre purchase inspections on mercedes cars are usually a recommended approach anyways unless you have quite a bit of experience with repairing mercedes diesels. In fact thats about the only way to know where to and what to look for.