The first step to keeping warm is keeping dry. Peacoats are a fall/winter staple for good reason. Find a good one and you’ll be wearing it throughout all walks of life. Cheap ones will fall apart eventually — pointless for an item you can expect to wear for decades. Just make sure the fit is flattering.
Dressing like you know what you are doing and dressing effortlessly are not mutually exclusive. Being crisp, comfortable and casual are not synonymous with dressing like the world is your gym. And there is nothing wrong with caring about how you look. Men’s casual is easy, relaxed and incredibly simple to set up.