If you want to take no risk that you'll ever have a problem, by a boat with a solid fiberglass
hull, like an Oyster
or a Beneteau
or any of the production monohulls.
If you want a boat which is lighter, stronger, and quieter, and with less condensation/heat loss, buy a boat with a cored hull, like Swans, Hallberg-Rasseys, and all catamarans.
My present boat is my first with a cored hull. The outer skin is Kevlar, and the core is made of encapsulated balsa blocks. I was quite nervous about this when I was buying
my boat, but the more professional advice I got, the better I felt. Apparently encapsulated blocks is a particularly reliable (and expensive) method which isolates the effect of any leak through the outer skin.
We shall see. I don't think it is without risk -- sometimes such systems just don't work out the way the designer
intended. As someone said, if a cored hull goes really bad, you're really screwed. Ask any SeaRay owner from the 1990's.
I can say, however, that the difference in strength is remarkable. My present boat's hull seems like the Brooklyn
Bridge -- no flexing, no creaking, groaning, or squeaking even under the most horrendous sea conditions, quite unlike my previous boat with its very heavy solid fiberglass layup
(4" thick in the bottom). You could hardly open the doors below in a seaway in that boat.
Also makes a big difference in speed. Cored hull despite being stronger weighs tons less. That gives my boat a racy D/L ratio of about 200, which really shows in the sailing qualities.
A guess a reasonable approach might be to avoid cored hulls in cheaper and older boats, but maybe consider them in higher end boats, after a thorough survey
As to cored decks, you have little choice. I haven't seen a GRP sailboat made in the last 30 years which didn't have cored decks. Here again quality is key. A well built boat will have plywood
reinforcements in the deck in highly stressed areas, and will be solid glass in places where there are penetrations. A poorly built boat, or one which has had holes drilled in the deck in random places, will have problems sooner or later.