All cars are outrageously reliable, except one issue with C63's, and the C63 is going to be at the gas station a lot. It gets the worst mpg out of all of them.
I know cause I used to have one.
It also needs tires every 5-8k even if babied.
Go check out MBWorld.
Also, the C63 has a known head bolt issue that will hydrolock the engine, and is very unavoidable unless you purchase a 2011+, which will most definitely be 50k+
I'd never own a C63 out of warranty, but I would on the ISF or M3.
Really it chalks up to this.
ISF - Most reliable under performer
C63 - Most expensive least reliable
M3 - Best performer, quite reliable, not bad on maintenance