[john70t]

Renting or a new home purchase?
That's an older unit, and if not cooling now, the damage may have already been done. They leak regardless especially if not run several times a year. The circulated oil conditions the seals and prevents leaks.
Not sure about the R-12 replacement costs/proceedures. All new ones use R-134A.

Look at the SEER rating label. Each number higher equals a significant improvement in efficiency. 20% or more i think. Depending on the zone and usage, replacement now will save a lot of money and discomfort.

There should be two tubes running into the house to the furnace, one hot straight from the compressor and one cooler/ambient return.