Check out these threads
http://www.hobiecat.com/community/viewtopic.php?t=9517 and
http://www.hobiecat.com/community/viewtopic.php?t=9466
Both of these have some good info.
If you look at the boat, you can determine the year by looking at the last 2 digits of the hull ID No (hand engraved), located on the upper transom near rudder mounts. I am pretty sure Hobie started making the Wave around 1994, about the time they phased out the old 14. The Wave was always roto-molded PE.
They are pretty near indestructable, damage to hull, if there is any, should be evident as cracks, if there severe impact or other stresses. Probably not common. BUT not repairable either, if severe.
At any rate, if the owner can't tell you the last time the rigging wires (stays) were replaced, or even if they can, plan on replacing them. These are not too expensive, really cheap insurance against a dismasting due to broken stay, which you don't want.