IME the only ones worth trying to rebuild in the W123 are the single-diaphragm units. That said, you can get a dual-diaphragm unit from Randy Steele (www.epsparts.com) for less than $50 the last time I checked. This is the kind of stuff Randy specializes in.

I just fixed the dreaded center vent actuator with a rubber diaphragm from http://autoluxmbz.com. The diaphragm was $5! He also has the popper for $10.
Seems like a nice guy who deserves some business, although many of you may have already determined that.