The evacuate should be done by an AC specialist (not neccesarily BMW) and if you were to invest into a set of pressure guages you could do this yourself. I have my own guages and have done it before and it's not difficult at all. But it is critical to get the system clean and decontaminated of moisture etc and the pressure settings are critical for peak operation. Plus there is a lubricating oil that must be added to the system anytime the system has been breached.
It boils down to investing $$$ into equipment vs just having a specialist do it occassionally.