Congratulations on your new boat! I'm sure you'll love the Hobie 18, they're a thrill to sail!
The good thing about the 18 is that most everything is replaceable or repairable, though the hulls are the most precious commodity. It does sound like you have a pretty worn mainsail. However, you'd be surprised how well even an old sail will perform. If you decide that it's something that you truly need to replace, you can find a lot of used Hobie 18 sails on ebay (in various conditions and at various price points), or through your nearest Hobie dealer (http://www.hobiecat.com/dealers/
), Hobie fleet or thebeachcats.com.
I would definitely lower the mast and inspect everything... mast, rigging, shackles, pins, etc. Look for wear, rust, cracking, deformation, etc. The recommendation is definitely to replace old rigging, especially if you don't know when it was last replaced. This is more critical on salties than fresh water boats, but the peace of mind that your rigging isn't about to fail is definitely a good thing to have. New standing rigging will run you ~$360. Again, you can get this stuff via your local Hobie dealer. There are also a few Hobie dealers online that will ship to you wherever, and there are always parts on ebay.
One of the best tools at your disposal in learning how best to fix and sail your boat is this forum. People on here are always happy to help a fellow Hobie sailor get better acquainted with or better at sailing his Hobie! If you find a local Hobie fleet, certainly don't be shy about introducing yourself and stopping by to ask for questions or advice. It'll shorten the learning curve considerably!