Everything in a wetsuit is determined by the water temperatures you plan on using the suit in. For water temps of 75-85F a warm water suit of 3mm or even less at times is fine. In temperate waters, 60-75F a 5mm suit is recommended and for waters colder than that, a 7mm or a 7/5 suit would be the choice.

Generally speaking, adding a layer to a suit can help, but you are always far better off getting a proper fitting suit of the correct thickness than you are adding layers of something to a suit, even a layer of neoprene over neoprene.