If you are going to want to go with more than 2 kegs, then I suggest either an old upright, or better yet, a small chest freezer. You can easily modify the freezer to work, and the only thing you'd need to use it is a temp controller.
Decide if you want the taps on the front (collar), or a tower on top. Advantages to both. With collar, you need to build a collar. With tower, you might not be able to open the freezer without it hitting the wall behind. Personally, I'd go with collar, even though I prefer the look of the tower. I think a frezer would be nice in that you dont' generally have to move one keg to work with another like you might with a fridge. Plus, routing the lines would likely be a bit easier too.