If I install a nice aftermarket head unit will I loose my steering wheel controls or will I loose the existing touch screen controls on the MFD?
You will keep everything on the MFD except the radio controls, but who cares since you have a new better head unit. You can keep the steering wheel controls with Amazon.com: Metra Axxess ASWC Universal Steering Wheel Control Interface (Black): Car Electronics I have it and it works great.
Agreed. Lots of info about aftermarket stereo installation on here (search for "F8L stealth") but I use this one with great success PAC SWI-JACK Steering Wheel Remote Interface : Amazon.com : Electronics. Good luck!