We were just talking about E500 wagons the other day: http://www.peachparts.com/shopforum/tech-help/338363-w211-e500-wagon-thoughts.html

Individual struts in the airmatic suspension do go out every now and then, but you don't need to do the whole thing at 100K, not even close. Probably it makes sense to replace both rear struts when one fails because there is so much common labor.

The E500 wagon is a great car, but you do need to be mentally and financially prepared to spend ~$2K on a repair. It may never happen, but then again it may. Just a cost of doing business at this altitude I am afraid.

Post up if you have any questions.