I believe this diagram represents your model:
http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=210020&M=606%2E912&GA=722%2E438722%2E600&GM=717%2E446&CT=F&cat=45P&SID=46&SGR=022&SGN=01
From what I can tell, the o-ring is #50, they call it a seal ring. I'm not sure, but this seal appears to be part of the boot jacket system and not the hydraulic seal, which is your root problem.
I'm troubled by your wife's attitude. Any problem on a car needs to be fixed. What constructive criticism does she have to offer?... or in other words, what's her position with your car? Does she just want to sweep this problem under the rug and hope the problem goes away?... not gonna happen. Or is her solution to "get a new car" as is the knee-jerk reaction of many women? Or perhaps she wants a car that is more utilitarian and less "hobby", as many older MBs can be.