I've not done this on a BMW, but compressors are usually a simple bolt on as are the driers. Are you sure the compressor is bad?
But there are some general issues about which you should be aware.
First, R-12 is expensive if you can find it. Most AC shops will have some, but many won't charge R-12 if there any leaks.
R134a is like $4 per 12 oz can. But you have to add special adapters which cost a few bucks. You would want to get a conversion kit for about $35. It'll have everything you need, except o-rings. You should replace any o-ring you see. And if you install a new compressor, you must add the appropriate oil and turn it at least 12 times before you install it. Unless you flush the system, you'l want to use POE (ester) oil.
Check the web for retrofit instructions, lots of good resources out there.