Congragulations on your new H-16, and your summer project!
Here are a couple of links you may find usefull, One is a parts guide, and the other is an assembly manual, just so you can know what you have or are missing.
If you need a place to find some used sails, E-Bay often has them listed, just search for "Hobie Cat"
Once you know what you have and don't have, the first thing I would replace on an older boat, is the standing rigging. By this I mean the forestay, and side stays. Then the Jib and Main Halyard. Since the boat wasn't yours you don't know what condition these things where in and how they where cared for before you bought it.
Hope you enjoy your new toy, and welcome to the Hobie Community!