Take the rear light covers off from within the trunk and pull out the trunk liners. Check for rust in the side wells. This often comes from water leaking through the rear window seals of from the rear tail light seals. The link is to the saga of my having to repair mine. It was a pita, but I learned a lot.
http://www.peachparts.com/shopforum/showthread.php?t=124667
Make sure all the windows work all the way. The window motors are kind of expensive, but you can get used ones for about $40 for scrap yards.
Another area of rust on the W26 is on the fender just behind the rear wheel at the bumper. This is how I fixed mine. Some people just slop some bondo on top of it and paint it over, but it will come back. If there was prior repair there see if it was done with metal work.
http://www.peachparts.com/shopforum/showthread.php?t=125002
Of course, as is important to any MB, check for service history.