Think about this:
Put the radiator near the ceiling and run a wide duct from there at least partway to the floor. You'll get a reverse chimney effect with the cold air that will run a convection loop with no fan.
Why power a pump? Use a hydraulic ram. Let the river pump your water.
(If you're less than 30 feet or so above the river the water won't boil in the return line, which means it will help pump the water, too, replacing the energy lost raising it. Then your ram only works against friction and you can get a lot more output flow for a given amount of water into the ram pipe. If not, a ram will still get the water up to you, as high as you want provided your plumbing can stand the pressure. It's just that the higher you go the lower the fraction of the input water goes out the high-pressure output.)
In addition to the crud in the coolant, radiators are (were until recently?) soldered with lead. They'll always leave some contaminant in the output water. Don't drink it. You might want to see if you can find a drinking-water-safe water-heating radiator. (Then, in addition to being nice to the fish and heading off legal trouble from environmentalists, you can water the plants with some of the warm waste water from your "air conditioner".)
