Nice looking TD. Iam not a BMW fan, so would use that as my beater car and baby the MB
as far as slamming the door to shut it. could be the striker as said above. if the rubber piece is missing. you can get one from the junk yard, but pull it from a rear door, they don`t get used as much. new ones are around $35.
When we first bought the 85 13 yrs ago, it had the same problem with the drivers door. moved the striker trying to get it to shut right. even replaced the latch mechinism from another JY donner.
one day I was down at the local indy and mentioned the problem. he had a little brush with some grease and lubed the latch. closed like a MB should.
Slamming the door will eventually break things in the door and or start cracking the sheet metal of the door. when I pulled off the door panel to replace the latch, I noticed there is a crack between a couple of the holes on the right side.
Charlie