I just did a kegerator a couple weeks ago. It was the easiest "build" project I have ever had. Realistically, it involves getting a 1" holesaw and drilling holes through the door/side of the fridge and putting in the shanks and faucets and hooking it up. No collar to build for a freezer conversion, no extra insulation to install. I can easily get 4 kegs in the kegerator. I would actually wager I could get 6 in there easily. But I have no plans to go for a 6 tap setup. I wouldn't drink that much beer in probably over 3 months.
Plus, you can use the freezer for extra food that you can't fit in the kitchen freezer. You could also move your beer mugs into the freezer so you always have a nice ice cold mug to pour into, freeing up some cupboard space.