Paying a premium for this car will be the norm, even for a used model. Make sure you check for track day wear and tear. Well maintained, low-mileage should the order of the day but these models will not be cheap. The best advice is to check out the Mercedes-Benz approved Used scheme.
The C63 AMG will fare well when it comes to selling on as there are plenty of AMG enthusiasts around. Service the model on a regular basis and keep it in a good condition and you should be able to get a good percentage of your cash back.