My C180 was sold at 11 years old with the original exhaust still intact.
Based on my experience, I would personally buy a genuine exhaust (or at least an exhaust made by the same manufacturer who makes them for MB, which is usually Tenneco).
However, if you're only going to keep the car for another year or two, then longevity isn't really an issue, just buy cheap and you'll be fine. In the olden days, Bosal was a decent budget brand, not sure if they're still around though.