I don't see why it wouldn't fit since the R171 is based out of the W203 chassis anyway, isn't it?
ESP and ABS compatibility shouldn't be a problem either since the electronics should be able to compensate for any discrepancies... plus the SLK55 and SLK350 have different rotors and calipers yet share the same Bosch ESP unit and likely the same ABS. If anything the C32 brakes are intermediary between the SLK350's and SLK55's.
As for fitting inside the wheels, the C32 had 17" wheels standard. I remember someone posting that the SLK350's sport wheels can fit over the SLK55's massive brakes to use as winter wheels. If that's the case, I don't see any problem with the C32's brakes fitting inside SLK350 wheels.
$750 is a great price if they are in good condition. Good luck, I hope the install works out well for you.