Most of the car radiators I have seen have plastic side caps, and an aluminum tube-and-fin core. I have seen large all-aluminum turbocharger intercoolers from 18-wheel trucks get crushed for their scrap weight. A truck had rolled and insurance was buying a new one. Although the original was only slightly bent, the mechanic didn't want to risk it having a crack (easily brazed) when a new one was included at no extra charge.
I think one of these might work slightly better than two car rads equaling roughly the same size (if one could be found at a reasonable price). I would mount the heat exchanger(s) flat and parallel to the ground with the warm air flowing over the top face.
Assuming you are mounting the reservoirs in the attic of your small garage, I would suggest placing them next to the wall in a tray that allows leaks to drain to the outside. I would recommend putting the heat exchanger(s) and fan inside the insulated room.
Your design allows the pump to push volume instead of pressure (small pumps such as these are cheap, and in your configuration they're easily replaced) and you also allow for the systems fluid expansion and contraction. A cup of chlorine bleach per 100 gallons should keep the biology experiments to a minimum.
The more wells you add, the slower each individual well will flow, so I very much encourage you to insulate all the cool up-tubes all the way to the heat exchanger. Slow uninsulated up-flow through the warmed higher soil will rob half your efforts.
Even with the heavy wall insulation you mentioned before, I would encourage you to shade the two sunniest outer walls. Five months a year I stapled loose weave burlap cloth (dark green and plastic fiber from "Home Depot") onto my southern eaves. It only hung down halfway, but shaded 3/4ths of the wall (plus windows) because of the angle of the sun, and it noticeable helped.
Running a small pump and a small fan will probably only draw as much as two light bulbs. Other people have done something similar with great success, but the ones I have seen are much larger and much more expensive. This is a great design!
This will sound crazy, but after you complete the system and get it working, one option is converting your pump to a DC motor (trash treadmill or Amatek?) and constructing an "Archemedian screw", shown here as a Vitruvian water screw (the tube and screw are attached, and turn together)
http://en.wikipedia.org/wiki/Vitruvius
I have seen them made from a coiled copper tube around a plastic pipe, lifting water from one tank to a higher one at about a 30-degree angle.
The lowlands of Holland were pumped dry past earthen sea-dikes to reclaim many acres of land with windmills turning these tube-screws (high volume/low lift). When there's sun, a PV panel can turn the water pump and the inside fan. If the DC pump motor has a shaft output at both ends, you can later hook up a small VAWT to it, so when there's wind, the pump will draw even fewer amps to turn.