What I did for cooling was I kept freezer ziplock bags full of frozen water in the freezer, and I would drop them into the reservoir (bags still sealed) as needed, that seemed to help a lot in a pinch.
the cooling system your talking about building, I remember kids in the frats used to have instant beer coolers like that, they had coils of tubing throughout the inside of the cooler, and filled it with ice. I'm sure it would work well for
nutrient fluid, maybe even if you could use the thinnest walled tubing possible, and as much as possible for the most amount of exposure.
Lighting...sigh...will always be an issue. Each type has it's pro's and Con's.
You might be able to get a lot of lumen from florescence over all, but the studies I've read say it just isn't a strong enough light source to penetrate through the plant. I always tell people, buy the most you can afford because if you buy 1 Flor-fixture, then another, then another..but the time you've spent enough to buy a 400watt
hps, you would have been better off buying the 400 watt to begin with. Of course, depending on your layout.