There are two basic offsets. The early cars had a small offset:
Pontons, Fintail, 108/109, 114/115, 116, 126, 123 and 107
More recent cars had the larger offset:
201, 124, 202, 140, 129, 210, etc, etc.
As offset change with rim width there are heaps of differant offsets.
ET44 is the dimension in mm front centre of the rim to the face where the hub bolts to the rim.
The Late Ones The Early Ones
5.5 X 14 ET35 6.0 X 14 ET30
5.0 X 14 ET50 6.5 X 14 ET30
6.0 X 15 ET49 6.5 X 15 ET30
6.5 X 15 ET49 7.0 X 15 ET25
7.0 X 15 ET44 8.0 X 16 ET23
So ET25 is quite small (25mm from the centre of a 7" rim). The later cars were something like ET44, that is a lot further from the centre. So if you put a W124 rim on a W126 the inside of the rim is in danger of fowling on
the steering arm and maybe even the brake lines - this is known in the trade as "a real bad thing".
The stud pattern has remained the same over the years, just the offset changed. Regarding spacers: check with the local regulatory agencies first. They're illegal here in Australia.
Some actual offsets of various Mercedes chassis
201 and 124 Mercedes offsets
The '84 201s came standard with a 14" 15 hole alloy 5" wheel (looked like the 15" wheels used on 124/126 from '86 on but was 14"). It had a unique lug bolt (not used on ANYTHING else that I know of). Part way into '85 they switched to a 6x15" wheel (changing to the same lug bolt used on the 124s). Except for not fitting on the 190E-16V sports cars, in '84-'86 you could swap these 14 and 15" wheels around. The introduction of the 190E 2.6 and 190D 2.5 turbo, both with vented front rotors and a larger caliper assembly (in MY '87 I think) the 15" wheels were redesigned and while none of the standard measurements changed listed in the specs changed, the newer wheels cleared the larger brake assemblies fitted to the more powerful of these newer cars where the older ones didn't clear! All of these wheels had 48-50 mm offset. The later wheels could be used on all of the earlier cars. All the US sold 124s thru the '80s used 6.5x15" wheels with ~49 mm offset (though the sportlines in the mid '90s MIGHT have used a 7 or 7.5" wide wheel - I have NO wheel listings for '90s 124s). None of these wheels should be fitted on any 123/126 even if they MIGHT physically fit! The '86+ 126s used either 6.5 or 7 x 15 wheels with 21.5 or 25 mm offset respectively. THEY can be fitted to older 126 and 123 cars but NOT to 124/201s