Deniss, I do not understand how you could have a license and still have such basic questions about AC... I suspect you have ' access' to someone with a license who will help you buy it .... (?)
You need to go to aircondition.com and read their ' knowledge base' section.
Your basic assumption about the hot air and low R-12 is faulty. There are all sorts of other things which can cause that symptom... but even if it is low you need to address WHY it got low before dealing with the charging side of the equation...