I was a little rushed in my post before, but essentially, the biggest things to look for on any beach cat is soft spots, water inclusion and bottom wear.
Soft spots are very common in older boats and in areas of constant compression & UV exposure, i.e. the top of the hulls forward of the rear crossbar, aft of the crossbar, etc. If you press on the hull, it should feel rock hard. If it feels spongy, soft, or makes a crunch, that's a soft spot. They can be repaired with epoxy and some drilling, but they'll keep showing up more and more as the boat ages.
Check the hulls to make sure they're dry. If there's water inside the hull, it could be an indication of a leaky hull, and it could cause soft spots down the road (if it hasn't already).
Check the bottoms of the hulls. If the gel coat has worn to the point that fiberglass is visible, the bottoms need to be recoated. This is a rather involved, but relatively routine bit of maintenance.
In addition to the big three there, and the other items identified in the articles I linked you to, keep an eye out for the following:
Solid mast or comptip? Solid masts are better for performance and less likely to leak, but are not race compatible. Comptips allow you to race, but they de-power the boat slightly, and if it's a retrofit, has a tendency to leak.
As I mentioned, you'll need the reinforcing brackets at the crossbars if you have wings. They're $50/pair if they're not already installed. Check the wings and crossbars for any cracking. These are heat treated aluminum, and cannot be welded.
Check the mast step on the forward crossbar. It should be riveted tight, with no movement and no gap underneath. If it's not, it needs to be re-riveted, or it will cause cracks in the crossbar.
Don't neglect the trailer. If it's in bad shape, you'll need to put some TLC into fixing it. Mine, for example, was extremely rusty, had rollers (not good for the Hobie 18), no working lights and no means of supporting the mast aft.
All in all, the Hobie 18 is a fantastic and versatile boat, and replacement parts are available for almost everything. Therefore, bad rigging or rudders or sails or trampoline are not a dealbreaker, but they may be indicative of the care the owner took towards the boat as a whole. The hulls are irreplaceable, so make sure they're in good condition. Crossbars, mast and daggerboards are very expensive and difficult to ship, so check those too. New sails and trampolines are also expensive. The rest is pretty replaceable. You can expect to replace the rigging if it's older than 3-5 years.
A note on painted hulls: It looks nice, but it adds weight and makes future repairs more challenging because it's not as simple as re-applying gel coat or buffing/polishing out the oxidation. Not much to do about it after the fact, however.
I know it's a lot, but don't get overwhelmed by it. The goal is to have fun, so in the end, just keep an eye out for the major stuff and resolve to fix the minor stuff as you go! It's all part of boat ownership.