Your local hunt club may have some specific guidelines as far as attire for meets, etc. Things like coat color, shirt color, etc. They probably require a hunt coat and ratcatcher shirt/stock with pin. It's best to find that out from your specific hunt club. I understand that some are very formal and traditional, and others are much more relaxed about attire.
As far as everyday riding and hacking, I like breeches and paddock boots with half-chaps. As far as a top I wear what's weather appropriate: a T-shirt in the summer, tucked in for safety, add layers as the temperatures require. A quilted riding vest allows greater freedom for the arms than a winter coat while still keeping your core warm. If it isn't a formal/semi-formal occasion or show, its all about functionality. Helmet as well, of course.