I will do some bits and bobs to it I wanted a M3 before but running costs age to costly for me, valve clearance and vanos faults and brakes etc is a big cost, I did check up on the merc and it I cheaper than the M3 from what I've found, I don't expect any performance car to b cheap to run
Valve clearances don't need doing on the e9x m3 to my knowledge, but did on the e46, likewise vanos hasn't been a major issue since the e36, sure some e46s had problems but nothing like as widespread and not heard of any widespread issues on the e9x either.
M3 isn't slower than a C63 on track normally, though it is quicker in a straight line I'd say.
Nurburgring C63: 8:13 E92 M3: 8:05
Top Gear Track M3 is quicker than the C63 Black never mind the normal version
I don't think M3 running costs are as high as you say, no longer have the old inspection 1, inspection 2 stuff like you did on the e46s.