If you are taking payments or information from website visitors via a form it should be https. On the google ranking we took the advise of google on our own brochureware site and found that changing from http to https caused more ranking issues than it solved. Looking back if I were to go again at the https on our brochure site I'd not do it. For an ecommerce site you have no choice. Https is also slower to render as passes through encryption before it renders.