Hi Mate,
You will be very happy with the reliability of an ISF, very few if any problems with them, maybe the odd water pump failure on early models but in general they are built like a tank. I also have had a C63, then went to a RS5 after that bought an ISF sold that and bought a M3 now I'm back in a 2013 ISF as I have started doing a lot of km again. C63 is faster and sounds better but no where near the reliability in my opinion, especially as they both age but I do prefer the seats in the C63 and M3. Sound wise the ISF is quiet so you will need to upgrade exhaust if you like noise, not a huge amount of options available though.
Try and get a MY11 ISF as it has the updated dash and daytime running lights plus a few other tweaks, it's worth the extra IMO. They really are super reliable and relatively cheap to maintain except for brakes but you can import your own parts at half the dealer price and as you said excellent bang for your buck.