The AUC sensor does not have to be replaced and really does not affect how well the A/C cools. The purpose of the AUC sensor to close the fresh air doors when pollutants (diesel, pollen, etc) are present in the incoming outside air. But, you can still manually operate the recirc mode with the button. The AUC sensor is located on the electric fan housing (top right I believe but could be thinking of anothe one of my BMW's). The sensor is a black plastic box larger than a match box, but smaller than a pack of cigs.
As for the A/C system, it was just low on refrigerant (may or may not have been Freon). The only proper way to correct a refrigerant volume issue is to evacuate the system and start over with the correct amount of refrigerant by weight or volume.