the compressor should be cycled by head pressure with an overtemp sensor
for safety. I would like to use a salt water cooled condensor on the high
pressure side and a freon gas that changes state around -40 C. Please

I'm not an expert, other than maintaining my 2000 bushel cooler for 20
years, but a good choice for low temperature freon is R-502. its used for
freezers. Hold on to your shorts when you buy some.

Karl