Can someone help diagnose what's wrong with my air conditioner?

The symptoms are:

• the air that it is blowing is not very cold
  • It's colder than the ambient air, but not cold enough to cool the house more than 1 degree
• there is condensation on the supply ducts exiting the furnace
  • The condensation is inside and outside the ducts
• there is water pooling on the ground below the furnace
  • I suspect this is probably the condensation flowing down the duct into the furnace then down onto the ground
• On the large copper pipe on the outside unit, some of the insulation had rotted away. I observed frost on the exposed copper.

I've tried:

• Putting new insulation around the outside copper
  • I have not observed frost since
• Put insulation around the ducts, but they still have condensation
• Cleaning debris around and inside the outside unit.
• Vacuuming the fins condenser coil and the evaporator, and used a alkaline cleaning product
  • neither were very dirty in the first place, and I was very careful not to bend the fins
• I run a dehumidifier in the basement
• I've opened all the registers

I have not tried:

• cleaning the drainage pipe
  • I can't access the pan or outlet from the inside of the furnace
  • the pipe is PVC that's been glued on. I'd have to cut it to investigate
• talked to a professional
  • I've got an appointment scheduled, but I always like to have an idea of the problem before they come so I can ask intelligent questions.

Other info:

• this same thing happened 2 years ago. At the time, the professional who came out added refrigerant and that fixed it
  • He said that if there was a leak, it would all just leak out again right away, but it took 2 years for the issue to come back.
  • Does that mean the leak is very slow? or is there no leak?
  • I've read that AC units don't just consume the refrigerant over, it's in a closed loop, so there is no need for regular recharges.
• the blower belt looks pretty worn, it's at least 5 years old, but it doesn't sound like it's slipping

Update

I had a HVAC company come out and take a look. The refrigerant was low. They gave me various options including finding the leak and refilling the refrigerant, or replacing the whole AC & furnace. The system was installed in 1987, so I'm ok with replacing it. Their quotes seems pretty high though, ranged from $10000 to $16000.