1. Your Hobie dealer sells the
neoprene rail kit replacement, page 11. It's cut to fit properly on your 18 and comes with a can of glue. Search this forum for tips on how to remove it. Basically, you carefully scrape off the old stuff by hand and glue the new stuff down. Be careful not to gouge the gelcoat and fiberglass.
2. Sail numbers are for identification in races. The numbers are assigned in increasing order as the sails are produced but have little bearing on the year of production since sails are replaced more often than hulls. Also, sail numbers can be custom ordered to whatever number you want. In your case, the low sail number likely means it is the original sail since 18 sail numbers are currently in the 17xxx range.