Just browsing the web and came across this interesting and quite comprehensive W123 only site - in French. Perhaps someone has already noted it here. I'll try to do better and come up with one in English next time.

http://www.mercedesw123.info

A few links to their pages that detail the various models.

http://www.mercedesw123.info/w-123/static.php?op=280E.htm

http://www.mercedesw123.info/w-123/static.php?op=200carbu.htm

http://www.mercedesw123.info/w-123/static.php?op=240D.htm

If you click on the following link, you'll be directed to their stack of online articles which have some quality photos that accompany the articles. Lots of info on SLS.

http://www.mercedesw123.info/w-123/static.php?op=nomenclature.htm

But, if you go to http://www.freetranslation.com you can have a free, instant translation of any page on the site. The translation is rough in many respects but will give the context of the topic more value to the English reader.