I know this is a somewhat older post but I was wondering what the OP finally decided? I've been considering both C and E55. My heart wants the extra power and room of the E55.
The E55 seems to have more potential for substantial maintenance bills but could also be just fine, depending on individual vehicle, history, luck etc..what At first the concern was Airmatic but this is really not a bad fix and can be a DIY. What scared me off were the issues with fuel tank leaks that require replacement of the tank. Lots of info on MBWorld forums as I've read.
I'm not super concerned about maintenance as I currently have been DD a '91 E34 Dinan stroker M5 for the past 2 years. Probably the best thrill car I've owned. I budgeted about $2k a year in normal maintenance stuff and do a lot DIY so "high" maintenance is all relative.
Now I want to get a new DD with good performance and descent reliability so I can park and begin more restoration on the ///M. For me, the C55 makes sense because of the perceived lower maintenance cost plus I can pick one up with 75k-85k miles for under $15k cash. Not bad for this type of performance.
So, if OP is still here, what did you decide and why? It may help in my decision as well!
'91 E34 M5
Dinan 3.6 Stroker, Tune, Exhaust, Sways