I've been thinking about getting a B180 CDI auto myself, and I assume you too are worried about reliability, not the driving experience.
It would seem that after about 40K miles the CVT starts to become prone to problems in the A and B classes.
It's said that metal wear particles in old ATF becomes trapped in the electronic gearbox valves and causes Limp-Home and other problems, so an ATF and filter change at this mileage can appreciable extend the gearbox life.
Some say the CVT boxes are intrinsically suspect, but a troll through this site will show a raft of gearbox problems on all MB models, i don't believe the CVT is any better or worse.
As with all MB auto boxes, I expect most go on for 100K plus and some, probably a small percentage, go wrong, but repairs are tres expensive.
So look for one with 25K miles on it and resell it at 40K while its worth something!
Cheers
Michael