Depends on winter conditions in your area. If it gets very cold, you're probably going to need double-glazed glass and more than likely some auxilliary heating. A car isn't going to keep out small animals like mice and rats. There's all sorts of ducts and vents they can crawl through, that you'd need to seal off first. If you take some care in building your greehouse, I don't see why you couldn't seal it up reasonably well against animals getting in.