That 116 looks like a nice car. Everything I've read says they handle sweet for such a big vehicle. You should ask the Boneheaddoctor how he likes his. Turbo 617. Nearly no blowby and no apparent rust. Best dash layout of all the MBs in my opinion. Sounds like a diamond in the rough. Can't believe anybody would poo poo it here.
I mean who really cares about that vacuum stuff when it's so easily diagnosed and fixed. 70's SDs are getting rare especially in this kind of shape. I do agree that the price is too much. Keep in mind that you could try to talk them down of course. Window regulators can be fixed. Radio? That's not an issue. Just get another one. It's got good paint! I see a nice car there, something I'd have for sure. I think you should look it over again and really look for the rust, and drive it of course. Unless that 115 240D is in extremely nice condition or you're emotionally attached which you probably are I would go for the SD. No I probably wouldn't so the only solution is to find a way to have both.
It's a nice 116 though, and think of how it would look with some Ronals and Euro headlights. BHD gets better mileage than my car gets with his so they're not hogs.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)