It sounds like you have a serious case of the cheap's, and I think you know how that tends to turn out.
It is an offence for unqualified people to handle most refrigerants.
With the line being discharged, does this mean I would need the line vacuumed to remove atmospheric air (moisture) before I use a DIY recharge kit?
Yes, it does mean that. In order to service an A/C system, you must pull a vacuum to remove all gases that are not the correct refrigerant (so basically: all gases). It is also absolutely essential to remove ALL water.
So you will need an A/C vacuum pump and a gauge set. This is not optional.
Are you sure you want to do this?
The latest trick I have seen in DIY A/C servicing is to draw a vacuum, then charge the system with canned nitrogen. Nitrogen is harmless, and does not hurt the system. Then, they hold the pressure on the system for awhile to confirm they have abolished all leaks. If it has failed, they vent the nitrogen and keep repairing the system. Once the system is tight, they take it to the pro to have freon added, or they use a DIY-legal refrigerant.
Would a drying additive work instead of vacuuming?
LOL no, that will not work.
Or is this something that can wait and be checked on later?
I don't see how. Even if you pressurize your system with something else for testing, you'll need to get and keep water out of the system.
I suppose you could pressurize the system with nitrogen just to make sure you've solved the leak problem, and then take it to the mechanic to have them vacuum it and recharge it with Freon, but that will still be a hefty bill.
Make sure to tell them it's nitrogen so they don't try to recover the nitrogen lol.
If the above additives are a good idea
LOL they're not. They're marketed as a "Hail Mary" play to people whose A/C just isn't going to work at all without them. The problem is, if they're moisture-activated, well your system is chock full of moisture, and everywhere there's moisture, that stuff will turn solid. That will be many parts of your system that don't respond well to having solids in them. It could do much more damage to your system, destroying all hope of ever fixing it.
I wouldn't touch any of that stuff with a 10 foot pole. As always, if it worked, the OEM would use it.
It's really that simple, forget those conspiracy theories about the manufacturer holdng out on us with secrets. The only thing the OEM cares about is the car making it through the warranty period uneventfully. They'll use any tech THAT WORKS, and no tech that doesn't work.
I believe they are to be added before the r134a recharge kit
It's illegal for you to use R134A. You do not have the certification, special recapture equipment, or supply relationships to get the recaptured stuff destroyed (or re-refined and reused; reuse is now the ONLY source of R22 for instance).
The kits are actually 12a which people keep telling me are the "same" as r134a (provided that you convert the math correctly)... but if that produces red flags I'd love to hear your all's thoughts on that too.
Propane is R290. It's actually awesome because it's non-ozone-depleting so it's legal to DIY. No cert, no recovery pumps. Only problem is, it's flammable. Same is true of R600a (isobutane). Your "12A" is a mix of those two, designed to behave like R12. Neat idea, but your system is tuned to R134A.
Every refrigerant has different ideal pressures, different vapor points, different latent heat. The system has to be designed and tuned for the refrigerant. R12 and R134A are different, and required redesign of A/C compressors and expansion valves. If you change refrigerants it won't perform as well. Well enough for you? Maybe.