Golf IV / Jetta IV :: AC Quit Working / Coolness Losing

Out of the blue, months ago, the A/C quit working. I rarely used it prior so I can't say for sure if it was losing "coolness." It may have been. And other than hearing a slight hum or whine after turning the compressor off, which it has done since almost new everything was fine. Although it did, recently, before the A/C died, begin to overheat while waiting in traffic. Never did it again.

I have not done a bunch of troubleshooting other than adding some refrigerant and monitoring the fans and A/C clutch to see if they cycled, which they did not (assuming I was reading the fan process accurately). The gauge did read low, I filled it to the green zone, the compressor clutch engaged, level dropped, I added, it cycled, then dropped, all the while still blowing hot air. I quit doing that but it remained on the low side of green.

The car has 180k miles and runs great. I'm getting ready to sell her after 11 years but really need the A/C working to get anything out of it in the South.