The two biggest places to look for W123 car in Germany probably are
www.mobile.de
and
www.autoscout24.de
For really cheap maybe you could pick up a TuV test failure car for a couple of hundred euros....
That is the source of a lot of them that wind up in East Europe, the middle east and west africa
Old Mercedes W123 never die...
The really nice ones around, in Germany, seem to be generating some interest there.
You really need to watch out for rust, though, especially in the northern parts of Germany
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1991 560 SEC AMG, 199k <---- 300 hp 10:1 ECE euro HV ...
1995 E 420, 170k "The Red Plum" (sold)
2015 BMW 535i xdrive awd
Stage 1 DINAN, 6k, <----364 hp
1967 Mercury Cougar, 49k
2013 Jaguar XF, 20k <----340 hp Supercharged, All Wheel Drive
(sold)