Adding sealants are a bad idea, if you value the continued operation of your ac system without extensive/expensive repairs.
Sealants gum up small passages in the ac system (like the expansion valve) besides sealing some leaks, making an overhaul of the entire ac system necessary.
Sure, if you're getting rid of the car, and just need the ac to work while you show someone that it's working to unload the car, it'll work. But I consider that to be a scam like throwing on fenders, and leaving the chassis and suspension dmage hidden under the shine.
It isn't necessary to fill the ac system with compressed air to find a leak either. Inexpensive (under $100) ac refrigerant gas leak detectors are available, as well as dye detection systems that work with the remaining refrigerant.
Yes, if the refrigerant pressure has reach/leaked down to zero, or you've opened the system to replace components or seals, you will need to evacuate theair and moisture from the system, BUTall you need to do that is a vacuum pump, which you can borrow for free from Autozone, or you can buy an air compressor (must be a full sized air compressor) powered vacuum pump for less than $15, or buy a vacuum pump ($60 on sale from Harbor Freight).