Well, we're talking master cylinders here, and we're also talking a stable of well-worn mules. Basically the wear items on MC's are the piston seals. If it ain't a-leakin' it's a-workin'. If your brake pedal isn't caving in as you maintain pressure, the seals are good.
The next question would be are rebuild seals as durable as OEM? No idea, but how long do they need to hold, another 18 years? Also, on my '85 W201 I remember replacing the master cylinder in 1997, so who's to say that OEM is better quality anyway?
I'm sure the re-man has a warranty, so I'd have no qualms using it, but would just make sure it worked to my satisfaction within the warranty.

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Prost!