You are better off getting a second complete bow as opposed to buying an extra set of limbs. Consider buying a used bow from the classified section. For a youngster like you who apparently is athletic, you would probably be better off going up to 55#, which is enough bow for any animal in North America when combined with the right arrows and broadheads, along with accurate shots. If you want to shoot lots of arrows for 3D competition, a 45# bow could be perfect. Eventually, you will want to have bows in all three weights. Take your time to figure out what you really want to do with your bows and then figure out what kind of bow is best for each use.
Allan