Get started with this thread...I've heard talk about aftermarket installations and amps cause you to lose Nav Voice and U Connect and maybe steering wheel controls on the 300C. Is this true?
Then watch me (an enthusiast) battle your questions on my car....
The watch El Duderino (an expert) go through it...
The short answer to your questions are
1) You have to add amps after the factory amp to keep your Uconnect. Most on this forum doing this are using an AudioControl LC6, LC8, or JL Audio CleanSweep as their line output converter. If you cut the preamp signal between the factory deck and factory amp, Uconnect voice stops working (ie controlled by the factory amp). The threads above go into great detail on this. As long as you keep the factory deck, the steering wheel controls will continue to work.
2) If you go with an aftermarket deck, you will lose the use of Uconnect with no known solution on the horizon besides replacing with an aftermarket bluetooth solution. You will lose steering wheel controls for the time being. There are canbus adaptors in the works for steering wheel controls but none publicly available as of yet.