I did this recently - got a chest freezer that fits 1 carboy/bucket with airlock and one without (so I use a breathable silicone bung for secondary), along with 1 keg and a bunch of beer. If I wanted to do secondary in kegs, I could fit 4 kegs (w/airlocks) and a 5 gal carboy (w/ silicone bung) in there at once. Actually, my plan is to get a bigger chest freezer for fermenting and turn this one into a 4-keg kegerator in the future. The shelf is easily big enough to fit a big C02 tank and a nitro/beer gas tank as well (so one of my taps could be a nitro tap). I'm not sure if I'm going to go the collar route or the tower route, as both have obvious advantages.
The bottom line: buy for expansion, because you will likely need it. On the other hand, any model that can be used for a standard kegerator (with a sankey keg) would be just about the right size for a single bucket or carboy.