You're correct that ferm temps are really important. Not that good beer can't be made under less than ideal conditions, but if you're anything like me, those first couple less-than-ideal batches are miles away from ferm-temp controlled batches. Of anything I've done since starting a year and a half ago, that and proper yeast pitching amounts have made the biggest differences in quality for me.
That being said, I started out with a swamp cooler (the rubber tub) and it worked great. The key part of that is that fermentation will generate heat, so if you put your fermenter in a room that is at 68 degrees, the temp of the beer is actually around 73-78 degrees. Just putting the fermenter in a tub of water allows the water to soak up that extra heat and keeping the temp near 68. Unless you're putting it somewhere cold, you probably don't even need a heater (generally we're more concerned with keeping temps lower, at least for a majority of beers). Do a search for swamp coolers and you'll find lots of other little tricks, like freezing water bottles to drop in, draping a t-shirt over the fermenter to wick up water to evaporate, putting a fan on it to evaporate more, etc.
If you want to step up to an actual chamber, your idea might work. I could see if you had a two-stage controller, one could control the light bulb and the other could control a fan to pull air from the colder conditioning section to the fermenting section. I'd check to see if someone has successfully pulled it off first, but it sounds like it might work. If you're only planning on having one beer fermenting at a time, you could consider a mini-fridge. I "borrowed" an idea from someone else on the board (think it was called mini-fridge fermenter if you search for it). Basically you just pull the plastic insides off a mini-fridge door, maybe build a collar if your fermenter still won't fit, and ferment in there. I generally leave a beer in there for two weeks, then pull it out to condition in the primary for another few weeks while the next beer goes into the chamber.