This is a perfect case of “There is no free lunch.” You got a good deal on the car because it doesn’t have air conditioning. If you want a car with good reliable air conditioning, you’re going to have to pay for it. There is no cheap aftermarket fix for this unless you want to cut a hole in the roof and install an electric air conditioner. If you do that, you will need either a generator or a high capacity inverter that is correctly directly wired to the battery.