I don't know what legal issues there might be, but I can give you some thoughts on the rest of the question. Plexi glass is really just plastic and will never be as clear as real glass, and it will scratch. It will be less likely to break, but unless you live in a war zone this shouldn't really be a problem. Single pane vs double pane? Double pane wins for energy efficiency and sound deadening, but obviously will cost more. Local weather and climate conditions would enter into the decision.