I've found that you can get substantial cooling even without a radiator - although I'm sure that would make the system much more effective. I've set up terraria with aquarium chillers using a makeshift radiator of coiled copper tubing or even plastic/rubber tubing (make sure to use opaque - preferably black) and they worked plenty well. Particularly if you use the potted-terrarium style and have a reservoir of water at the bottom.
I never got a chance to verify that it worked better, but the last one we did had a reservoir made from a small ice chest in the cooling loop; the chiller pumped into the terrarium, and then the water exiting the terrarium went into the ice chest and from there drained back into the chiller, the idea being that it would store some of the coolness even when the compressor wasn't running and help curb rapid temperature changes. It was a very nice chiller - built-in thermostat, ~4000 gph cirile capacity - so there was little need for improvement, but the compressor did seem to kick on less often when we added the cooler.
I want one bad. That monster could hold 100 gallons of water at 50 degrees in mid-summer... glad it wasn't my electric bill though. XD
Welcome to the forums BTW! (If I haven't already said.)
~Joe
PS - Re: controlling it, if your chiller saves it's set point when powered down, then the external thermostat could potentially help fine-tune the cooling process. Or you could use a closed loop on the chiller to cool the reservoir of a makeshift swamp cooler, for example, and then blow warm air from the lights into that to produce cool, humid air. You could also use the chiller loop to cool standing water in the bottom of the tank, and use something like an airstone or a small fountain pump to agitate and circulate the standing water, which would have a similar effect without requiring a wick and forced air. Being able to separately control the temperature of your cooling loop and the set point of the rest of your contraption should make it possible to reduce the strain on the chiller while giving you a say over how fast things cool down when it all comes on. By my understanding, the really nice thing about those ZooMed gadgets is that they allow you to have separate profiles for day and night temps/humidity (with day and night determined by a photosensor.) Swords recently set one up and wrote a nice thread on it - should be on the top page or two of the terrarium subforum.