The factory air in a stock SC is borderline when working correctly.
The a/c in 86 Carrera have the larger vent openings in the dash and work much better.
I have a 82 SC. 4 years ago I redid the a/c. New rotary compressor, txv, reciever/dryer, and complete line set. Cost was ~$1300. I did all the work myself, a little over 2 full days. Removing/installing the line sets were a major PIA. I did all the little things, remove front condenser, evap coil and cleaned them, cleaned drain line, tried to seal ducts. System is R22.
I would guess a shop that does this stuff everyday and is familar with Pcars would take 10-12 hours minimum (get it in writing
).
Don't confuse air cooled Porsches a/c with a Honda/Ford/whatever. The a/c in a air cooled car will not cool in traffic as well as a water cooled car. The a/c in the SC is more than adequate around town or on the road. Stop and go traffic is a killer.
The a/c in the car you are looking at has not worked properly in 15 years. Plan on replacing everything I replaced. Bottom line, DIY for around ~1500 and a lot of skinned knuckles or pay a shop ~3500 if they supply parts and labor.