If you can find an old add-on AC (aftermarket like FrigiKing) you would be well equipped to make a reasonable AC. Otherwise I can't imagine installing the evaporator box and all and making it factory air. Unless of course you had a donor car and liked to pull things apart. To do this you remove the dash, that means pulling the front window out. Just that is about $300 including gasket and reinstallation, no A/C parts. I would use a Japanese compressor, not the old York, there's brackets available to use this on the W115 chassis. Finding all that is another story.
Honestly, this may cost you $2500 or more.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!