W126, Used Parts Assortment

I recently got an 85 300sd with an engine that only has 92000 on it. The car drove great till I heated it up on a 30 min trip home from work during the hot part of day. I was just about home when I turned on the A/C, ahh it was nice and cold, but at a stop light the engine started to make some strange vibration and a few minutes later coolant vapor burst into the cockpit (not very nice while you are driving). I got the windows down and pulled over and the vapor slowed down, but the engine temp started to get hot.

I was getting things together to get started, new drier, new expansion valve, refrigerant and 'oil..' I was going to draw it down today-- and get to work this afternoon.. My freeze-12 (drop in "Freon" replacement) arrived a couple days ago.. Here's my problem.. Uncapped the lines, to find one has been "retrofitted" to 134a. My questions are: 1) can I just wrench that off?? It looks like it's just a screw on fitting. 2) the BG refrigerant oil is compatible with both 134 AND R-12 "freon" so I'm thinking that isn't an issue.. 3) do I need to replace the receiver drier and expansion valve, and pretty much, go on with my life??