General A/C questions - missing some components

I need to get my factory under dash A/C working with my sanden 7176 compressor and aftermarket condenser.
Currently I have:
1-Everything under the dash, though not wired, and vacuum lines not hooked up. 1973 Plymouth Duster factory dash.
2-Compressor
3-Condenser

I'm going to start working my way through the system this evening. Get the blower motor wired and running, and play with the vacuum system.

Questions I have that I wasn't able to readily find answers to:
What is it on the AC line near the firewall? The manual lists it as an expansion valve - what does it do?
The manual talks about the anti-freeze switch- where is that located?

That's all I've got for now for questions. I know I need a new drier, low pressure cut out switch, and custom lines. Anything else I'm missing? Bouchillon Performance sells an AC clutch cycling switch that they say is needed for a Rotary Compressor - I'm guessing I need one? How do I go about installing one?

Thanks!
Wade