Is that deep enough for a six-gallon fermenter even with the extension? Depth wise, mine won't fit anything deeper than a six pack on the bottom. My fermonster is pretty big. If you have it inside, it would be easy to control an ale. In the garage with a lager might be a different story.
I bought a normal, used refrigerator from a friend for cheap.
I did see a chest freezer for really cheap at Best Buy the other day, but inserting and lifting a full fermenter from the top might be a bitch with one of those.
Not trying to be negative. I actually think it is a cool idea, but make sure you measure the fermenter. Also consider the number of bottles you need or the size of the keg if you are kegging. Bottles carbonate at the same temp as an ale. The height of my corny keg barely fits in my normal refrigerator.