Aaron, Not sure the exact amount of duty on used items. A lot will depend on the customs agent you draw. Some can be lenient or they can give you a hassle. Be sure to tell them all items are for your personal use and to remain on the island. Make a list of your things (don't worry about clothes) to show them if necessary. You might want to rethink a large collection of knives and swords if you are using them for decoration. Not only a hassle to bring down, but if someone breaks into your house , it would be the first thing to go. You'll find all kinds of interesting things and art in the country that could be the start of a whole new collection and a nice reminder of your time in Belize when you decide to return to the U.S.
Everyone on the board recommends a copy of "Don't Stop the Carnival" for any newbie coming down. Good choice and must reading, but also buy yourself a copy of Dr. Suess, "Oh, the places I'll, the Things I Will See." Your young and starting a new adventure make the most of it. Judy