Spray doesn't need to function continusly. Install 50C electric thermostat on condenser outlet to control spray on/off cycle. When the gas is not being effectively cooled by condenser, it is indicated by high discharge temperature causing the thermostat to flip on. The spray duration will hardly last 5-10 seconds but will bring temperatures considerably down. 5-7 liter water jacket should last you full day on a proper spray system.
If you have a an AC tech to assist, you can route the refrigerant return line via water jacket. Waste cooling of return line will chill water reducing consumption and improving efficiency.
Regarding water pumps, high pressure head lamp washer pumps from 4x4 are available in shersha for 300-400 RS. Just don't try educating the guy or he may ramp up the price.
R-290 is only sold for commercial purpose therefore may not be available in small quantity. Another alternate to R12 is R12a. Being an HC blend, it exhibits properties similar to R-290 but is available in small containers.
- Lower head pressures
- High latent heat capacity which leads to increased air con performance. Especially noticeable in peak hot season.
- Less power drain on car engine.
When converting from CFC to HC blend refrigerant, there is no need to pull vacuum. You can add HC a top of existing CFC and they will mix. When charging by weight, 3oz of R12=1oz of R12a so do remember u only need to charge 1/3 of required R12 capacity.
A word of warning as usual, do remember all HC refrigerants are highly flammable so safety first always.
If it interest you to know, R-290 is really refined LPG (pure propane). You can charge LPG in your AC but its not really refined up to refrigeration standards therefore any excessive impurities may destroy your compressor.
BTW, mounting a condenser on roof affect aesthetics and kill your aero dynamics Resultant drag created at cruising speeds may be enough to take off Mehran's roof 
