1. Hydration pack is god. Believe me, they make the difference between a sluggish, dehydrated day and a fun, active day. They are worth every penny, and honestly, aren't really that expensive. I put ice in mine during the summer, and it acts like a portable AC.
2. Consider holding off buying the vest for now. Instead, find as many occasions as you can to try as many different types of vest as you can. In my experience, a good vest is first and foremost about comfort, with carrying capacity coming in second.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."