2003 X5: A/C not blowing cold air

Today I found out (issue is also verified by the BMW dealership) my A/C is not blowing cold air. Since my car is out of warranty, I am trying to find a cost effective solution. I did a little search. Some posts here (regarding this A/C issue) say that if my outside temp sensor functions normal (it shows 72 degrees, rather than -40 or something), it could be a Final Stage Resistor issue. Other posts say that getting a can of r134 from Autozone can easily fix the problem. I also heard the term freon leak. I am confused. Can anyone help? I am pretty new at this and I would really appreciate if someone can guide me. Is this a major repair (need to find an independent shop, then) or a 1-2 hour fix (can be done at the BMW dealership)? Thank you so much in advance.