Vac pump is currently $175, although two months ago all the vendors had it for $275. Belt tensioner lever arm is $50, and the is shock $30. Fan clutch is about $250, radiator is ~$300 for new or ~$150 to have your original rebuilt from Reseda Radiator. If you buy a car that already has these items replaced, the only thing you need to watch is the belt tensioner, and the temp gauge in the summer. FYI, with all new cooling system parts, the temp gauge should never go over 100C, or 105C in extreme conditions. If you test drive a car that exceeds those temps, especially if it's not that hot out, it has a cooling system issue - which you can use to negotiate on the price, if you still want the car.
The trap oxidizer appears to be a partial cause of excessive heat load on the head, but the head cracking could also occur if the car overheated without the trap too. It's not THAT common but you should at least be aware of it. A new head will cost $2500-$4000 installed, depending on the shop.
The 1986/87 SDL's all had the same engine. The 1981-85 SD's all had the old iron head engine. There are no iron-head SDL's and no aluminum-head SD's (at least in the W126 chassis, not counting the 1992-up W140 chassis, which is $20k+ used).
The 300D is smaller, lighter, and looks similar outside. The interior is quite different, and yes you lose the rear leg room. Take a look at any 1986-1995 E-class for a size comparison, only the engine and cosmetics are different during those 10 years. And no, there was no 1986 model 300D in the USA - if you find one, it would be a gray-market import (and I don't think they exist - MB put a non-turbo OM603 in the Euro cars!)
There's a post over at MBZ.org about buying a 1987 300D, I wrote the first part, and Doc Booth did the second part (note his comments about the 2 sound panels, they cost roughly $200 each!)
http://www.mbz.org/info/articles/misc/used124/
Regards,