I currently own a central air and heating unit that has refrigerent-12 in it.
My unit stop cooling, and I'm told I have a leaking coil.
I'm also told that I need to replace the total heating and cooling system because one can not buy unit that uses refrigerent-12 anymore. I need to know the cost of replacing a total unit today.

Its really impossible to give a quote without seeing the job first hand. I suggest getting as many quotes from local installers as posssible. Good luck and please let us know what they have to say.