They aren't a whole lot, you can usually find them used on the bay for about 200-300, new they run about 400. But yes you would need one since the one you have now would only have the input for the rear camera, and you would need the input for the side cameras. Part #'s can be
found here.
From what I've been told it is possible to do just the side view without the top view, not sure how it would look on the display though. You can drill the holes for the mounts I believe, and you should be able to find most of the part numbers for everything on Realoem, and of course you would need to code it to the car once you got everything installed. If you have any other questions or need help with anything let me know.
