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So I've been fixing things to get my AC working, and I'm on a roll replacing bad a bad Air Mix Actuator and Blower Motor, and now I can blast neutral air from all vents. (Constant, HOT air last summer was brutal).

Years ago, the vehicle sat unused for like 5 years, and at some point the freon went low enough that the compressor stopped clicking on. My guess was this was years ago. So the ac has not been used in years.

Is it safe to say I'm going to need to replace the compressor and/or clutch? And/or everything else related to it? I have a freon refill kit (with dye), is it worth recharging it and seeing what happens / finding leaks if necessary, or is that just a waste of freon? I'm fine wasting it if there's a chance it will just work, but if it's 99% that it's completely shot, I won't bother.

Most likely you have a leak. The compressors tend to be the culprit on GM products. They leak around the main seal in the center that seals the two halfs.

The clutch may also be seized. However, it also may not be. 50/50 on that depending on where/how it sat.

You can check if it had a leak by looking at the bottom of the compressor and seeing if you see oil pooling around the center seal i was talking about at the bottom. If you don't see any, charge it up and see what happens.

Would the oil be pooling there even without any freon in the system?? I want to make sure I would be able to see that BEFORE I recharge it, as opposed to after, so I know what I'm looking for!

I would also give serious consideration to changing all of the o rings, the receiver/drier and the orifice tube or inline filter.
As tytotheler92 said, compressor seals often leak but also changing all the o rings is a low cost, effective way to be sure the system becomes leak free.
The receiver/drier should be replaced when the system has been open, especially for so long and the orifice tube or an inline filter typically traps debris.
Just so you know, it is illegal to discharge refrigerant into the atmosphere because of what it then does (R134a is a "greenhouse gas" and is being phased out), so it's best if you can be sure the leak is truly fixed first.
A reliable way to check that leaks have been fixed is to charge the system with nitrogen, which is harmless to your A/C and the atmosphere.
Barring that, a less reliable way is to evacuate the system and see if it will hold a vacuum.
You probably already know that bringing the system down to a vacuum removes moisture from the system.
Hopefully, we won't all be forced into using the flammable refrigerants that they're using in Europe, such as R1234yf---which seems like a giant backward leap safetywise. http://www.vehicleservicepros.com/blog/ ... efrigerant and http://www.autonews.com/article/2013123 ... z2pFqcT9dt
Call me narrow-minded but I believe that a pristine earth isn't nearly as good if many of us have to die to get it that way.