Totally agree, I had a mehran once and AC was not a good experience I am not sure about the Factory fitted but I don't want to discourage you, go for it.... I don't believe the AC can chill you but a pleasent feeling,
Tint the windows this will make the inside cooler too and put less pressure on the AC and when parking the car a keep a little windows down, put up the screen cover for the wind shield any thing to keep the car cooler from the inside, avoid parking directly in the sun.
- The battery and alternator are in good shape
- Radiator - Corolla , Charade , Double - Key is PLASTIC TOP ( might have to modify the front)
- Good Condensor with FAN ( like a 2.7 T Audi)
- Install a Thermostat
When the car goes for Service make sure the guy does not pressure wash the Condensor, the pressure will bend all the FINS will reduce passing of air
I don't think CNG and AC are a good combo , I don't know