MAF sensors
Just assume you will be replacing the MAF in around 20k miles or so. Mine kicked out at just over 78k; it gave me just enough range to drive home from Munich to London, and then once it was parked in it's own garage slot and the mechanics were coming to get it, that's it - no chance of starting again.
The MB 5-speed transmission isn't that long-lived either - and most annoyingly, it fails in a quite sudden and un-signalled way. If it takes more than a second to shift from neutral into drive and you can hear "ringing" or circulating noises sitting still with your foot on the brake then it only has a few more shifts left!
I haven't felt any sign of that yet in the 55 - I'm hoping that this transmission is built differently to the one in my 43, which doesn't audibly back off the throttle on upshifts like the 55 does.