The 'no air through the center vents' is a common problem on the 124 body, but I cannot remember what the cause is. If you will do a search for this I am sure you will turn up a lot of info, though, because almost every 124 has this problem sooner or later.

The 1987 300D has a problem in that if the head cracks there is just no fixing it and you will need a new one. You might want to price one of these before you buy the 1987. I have also heard that the turbos that year had a recall issued, but a quick check with Mercedes can confirm this. You will need the VIN and a dealer can tell you everything that was ever done to the car if it was done at a dealer. Like a recall item.

The 2.5 is a good engine but the vacuum pump needs to be replaced at about 150,000 miles. This is an easy job but the part is about $250 or so. The head gaskets on these tend to let go at about 210,000 miles, or so I have been told by several Mercedes mechanics. Repair on this is about $3,500.

If I were you I would take what you have learned here and then repost the question in the Diesel discussion area. And don't forget to bring up the 'no center vent air' problem there.