The hardest piece is...
As Twistinglane said, there's a lot of info in my book (see my signature), but it's general information about retrofitting a/c into a vintage car.
I did a full a/c retrofit on my '73 CSi. Long story short:
--Buy an original evaporator assembly (they're around, and the same as a Bavaria).
--The hardest part to find is the plastic piece that sits on top of the evap assembly and directs the air through the grills in the center console. They're plastic, unique to the E9, and incredibly fragile. Someone else discovered, I believe, that the Bavaria piece will work if you block off the side sections.
--Buy an air-conditioned console and faceplate. This is unique to the E9, and rare. If the gentleman above has these pieces, throw money at him until he sends them to you.
--Then, basically, throw out every other a/c piece.
--Buy a new rotary-style compressor.
--Buy the bracket to adapt it to the M30 engine. This used to be trivial, but the adapter brackets are NLA. Several long threads on this, including one of mine.
--Buy the largest parallel flow condenser that will fit in the nose, and the largest fan that will fit on the condenser.
--Make all new hoses.
--Install a new receiver-dryer.
It's very do-able. I love having working a/c in mine for the long drives to and from The Vintage.
--Rob