No matter what car you look at, buying used carries risk. I know people that tell me that they would never consider a used M, purely because of how they think people drive them, which of course is a generalization. CPO is a good option, but dealers sometimes don't CPO an M just to keep the cost down. Be careful about assuming just because it is a BMW that it will have issues outside of warranty. It's all about how the car was taken care of. I have owned two BMW's out of warranty and never had any significant issues with either of them other than normal wear and tear, but I took very good care of them.
The 335 is a great car and does very well as a daily driver. In the end, you have to decide what you want day to day. Personally, my next car will likely be an M, but I also don't drive significant miles and I have a third car, so some of your concerns don't really come into play for me.
In regards to issues. Can't really speak to that. However, remember that in '09, there were some significant upgrades to the Nav and iDrive.
Current - 2009 Carrera 4S
Current - 2010 128i convertible (wife's car)
Previous - 2009 M-sport 335i coupe
Previous - X5 3.0i with prem/sport
Previous - 2000 ///M roadster
Someone should invent a sarcasm font...
"In car rock, paper, scissors 911 beats Prius everytime."