Please Help me if you know

Hello everyone,many thanks to all friends!

I have another question,it's about diagnostics,
software diagnostics.
I will in future work with a/c systems (air-conditioning in cars).I have computer (notebook),but I don't not which
program to use for European cars,USA cars,Asian cars....?
Because I will work with a/c system,I have buy books about a/c system and learn much of these systems.
I have many of tools,will buy more,but haven't software
for diagnostic cars.
I know for OBD2,will learn more and need help about
software.
Do you know what software to use for Volkswagen, Mercedes,BMW,Opel, Peugeot,Ford,Nissan,Mazda.....
as many as possible cars from 1995-2008 or 2000 to 2008 years or any years for old and new cars.......
If don't know please help me to find who do or where do I look for theese informations.
(I bought PCMSCAN software but this software only works with special cars(gasoline from 2000-2008,diesel
from 2004-2008) and with car which have only OBD certificate,ok I cover these cars but I don't cover many cars from 1995-2008 or 2000-2008 in one word
please help me in order to find who know about these problems.If someone where do I post this thread if I wa wrong section?

Thank you very much !