I can feel for al of you. My suggestion is, honestly, keep the aging Lexus and get the 'other' MB Diesel on the side. It is obvious, that MB has lost out to Lexux beginning in the late 80's due to quality issues, I firmly believe that LEXUS is now producing a car with much better quality and reliability than MB at the same time. I hate to make this statement, but I know that quality standards at MBZ have gone way down after the intro of post-126 models.
Even as a die-hard german myself, here is my judgement: I will collect as many W115, W123, W116 and W126 as I can (as much Diesel as possible0, because those represented the proper balance of extremely high quality and durability. The turing point was after W126
If I were you, get the Diesel you want but keep the Lexus, share the opinions in the forum openly and then part after one year -if you need to- with either the Lexus or the MBZ