I am gonna take a trip to Europe next week and I planned to
bring back with me a pair of used headlights,to replace the
stock(ugly) seald beam on my 82'300SD...they are almost
free in Germany(the headlight assembly).My question is: do I
need any special clips or modification to install them or they will
just fit in?

I have been going to europe for long periods of time each year.... not for anything to do with MB, but other reasons/pleasure. Anyways... I find it that euro headlights for w126 seem to be a RARE find... and if yo do find them.. are as costly as ones yo can find in the US. They should fit in the car with no problem.. just that the passenger light will need a bracket at the bottom.

You'll need the Euro spec electrical connector but that's available from any US dealer.

Sixto
95 S420
87 300SDL