Greetings from Chicago!
I highly recommend going with an AC unit if you want to build something to purpose for multiple temperature zones. Especially if you're talking about using one compressor to cool two separate areas, I'd expect that an AC unit would do the job best because it has a big ol' fan in it pushing the cool air around. You'll need the fan in the AC unit plus some extra fans to move cold air from one zone to another. Besides, a 5,000 BTU AC unit can be had for next to nothing compared to a freezer.
Naturally, you won't have to build anything if you buy a freezer, but if you expect to be dealing with 50 gallons of brew in carboys and buckets or similar fermentation vessels, the dimensions of most commercially available chest and upright freezers and refrigerators won't be optimal. The space might seem sufficient, but the problem is how the space is laid out in most of them.
Here's something prrriiide did that really fits the bill:
Planning stages: http://www.homebrewtalk.com/f163/fermentation-chamber-build-222380/
Building stages: http://www.homebrewtalk.com/f51/fermentation-chamber-build-239480/
If you decided to go with refrigerators or chest freezers, it would probably be a lot easier to buy two and just set them for different temperatures.
Just my .02. Building a whole new refrigerator from scratch is for factories. Foam insulation sheets are relatively inexpensive and do just fine insulating.