Lets see some detailed pics of the boat in question
"stored out of the weather since new" is MEGA RARE! Any hobie thats been outside baking in the sun for 25+ years can delaminate... I've seen late 70's boats that look like new and late 80's boats that barely float. Depends on a lot of factors.
Get the newest boat you can find/afford.... hobie made a ton of small yet very well thought out upgrades to boats in the 90's and forward. If you plan on racing, you need a 2000 or newer boat for the stiffness, tight hardware and great crisp sails. IMHO. The core foam material shouldn't worry you much if the boat you're looking at is in great condition.