What Comodave said - decks, windows and fuel tanks. If the boat is all original, and if it has spent its life under the sun and stars, then those issues should not be hard to spot. If you don't see them, your surveyor will.
Its kind of weird - for decades, GBs came out of the yard with some of the most impressive build quality of any production yacht on the market, and yet many of them have aged poorly. Unless you are lucky enough to find one of that vintage that was either 1) professionally maintained with an open checkbook, 2) maintained by an owner with the time, tools and skills to stay ahead of the usual troubles, or 3) stored in a barn since new, you are likely buying a project. What's especially frustrating is that a Grand Banks that has been maintained in Bristol condition is one of the most beautiful, satisfying boats to be found anywhere, but a GB that has been neglected is heartbreaking.