Will 15" wheels from a 126 chassis SD fit my '85 wagon without adaptors? Can I just bolt them on? thanx

Yup, provided they are the correct offset, no spacers necessary (or desired!)
The 15 in wheels I use are described as:
Flat Face chrome wheels p/n 126 400 4102 from 420SEL etc.
A W123 wagon will handle better, esp with decent tires, with 15 in wheels mounted on it. It won't handle anything close to a 124 chassis, but better than with stock 14 in wheels.

Unfortunately finding good 205-65-15 tires (the correct size to use to prevent significant speedometer error) may prove more costly than you'd like, especially if using Michelin (I do not recommend Michelin).
Pirelli discontinued the P6000 (92V rated tire) that I like for the 300TD, although this was a softer compound than 94H used with P400 tires, it really handles great. The P400 really lasted longer but they also dropped that in this sizes too darn it!
You might try the 205-60-15 Nitto NT460 if a slight error in readout is tolerable. I really like the Nitto's on my 124.
DDH
Dieseldiehard
1971 220 (gas) 4-spd manual 106441
1979 300TD w/ ’85 turbo engine 297150
1983 300D 242300
1985 300TD 212700
1987 300DT 257K

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

I didn't say, but the wheels I was referring to are 15" bundtcake style. thanx D

Are you sure they are bundt 15 in wheels? those are very rare, made for the 6.9 W116. Flat face 15 in wheels are much more common. The Bundts would fit I am sure, as they only came in one offset, while flat face are available in many offsets and widths.
Beware that sellers of wheels on eBay have been known to measure the OD of the wheel to obtain it size, neglecting the fact that the bead is about one half in each side, and thereby misrepresenting wheels as 15 in when in fact they are 14 in. If you found real 15 in Bundts you better grab them if the owner has them priced low.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

On my wife's 82Sd, we are running size 215/60/15 with NO CLEARANCE problems...........they are virtually the same diameter as the 205/65/15. As a result, the speedometer is accurate.
see 'ya.
spo out

When I had my Miata I stumbled across this interesting little tool for calculating speedo error based on tire size. Granted, age will give some error but this is most helpful for getting close when changing tire size.

http://www.miata.net/garage/tirecalc.html

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Rick
Avid Benz watcher

Previously owned 126's: 2 1981 300SD's, 1987 300SDL, 1991 350SDL,
Previously owned 123's: 1978 300D, 1983 240D, 1985 300D
Previously owned 124: 1994 320TE

Currently:
2004 Toyota Sequoia SR5 206k
2010 Ford Edge 122k
Always looking for the next MB diesel!