I'd certainly go with a complete manual heat-AC system out of a 240D.
I got a complete manual heat-AC unit with control panel out of a '79 240D at the Virginia Beach Pick-N-Pull. It was only $12 (half-price day!) and someone had already pulled the dash, so all I had to do was unhook some wires and vacuum lines, and carry it out. Unlike most Asian vehicles, this W123 Mercedes unit doesn't look modular. Without inspecting a non-AC W123, I can't really tell if it's possible to install AC in one without exchanging the whole heat-AC unit, but it doesn't look like it.

Other considerations for adding AC are - will you need the York compressor, bracket & hose setup, as mounted on the upper right side of the engine on early W123 240D up to 1980,
or the Delco-Frigidare R4 compressor, bracket & hoses mounted low on the left-side of the engine, as used on later 240D and 300D/TD?
The York compressors are primitive and noisy, but the plumbing looks easier.
The Delco R4 compressor is smoother and more effcient, but they're not famous for durability, and the plumbing wraps around and under the engine.

ALso, will you be running R12 in the AC system, or updating/converting to R134a?

And don't forget the aux electric fan for the condenser.


Happy Motoring, Mark

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DrDKW