Buying a freezer for fermentation

I thought ales would be enough for me but I really wanted to make a great Oktoberfest but I don't the right fermentation temps at my house. So I'm going to get a freezer. I might get one on craigslist or I might buy new. But what specs do I need to look for?
I know it has to be big enough to fit a carboy but what else am I not thinking of. Do I need anything special besides a new thermostat?

Great question. I'm in the same spot.

You're set once you get your thermostat. Size is up to you, but bigger is better in my opinion. You can always store your hops in there.