rebuilding Rear A/C and converting to R134a

[Cumminpwr11]

I have a vintage air setup and rear ac. I was told that the front A/C doesn't work is because the systems are different. I didn't know that R12 had to have anything special done to run R134a. The rear fan blows, but doesn't get cold. My vintage are made a few noises like something was loose by the fan blades then quit working all together.

Going to have the vintage air looked at, but was thinking about getting the newer gen 4 so I don't have to fight with the cables when switching from cold to hot.

Just looking for info on where to start. Not sure if the mechanic that looked at it was full of it.

Chris