I'd say a direct answer to the original post is the Ericson had the best original build quality and will "age" best all things being equal. ( Things may vary here in the real world, batteries not included, local restrictions apply)
I would agree. OTOH most any production boat can be a great protected-water cruiser.
One thing that is more important than mere "age" are key construction engineering techniques -- like keel to hull attachment and hull to deck attachment.
As designs age, having the deck and hull glassed together with roving (Ericson) becomes a better idea with every decade.
While it's quite possible have a solid joining with a "coffee can" scheme, given the right adhesive and rivets, it will always be less robust and more leak-prone.
Besides the Ericson standard with roving joining the top and hull into one piece, look for a full-overlap on an inward flange on the hull side, like Niagara, Nonesuch, C&C, and our Olson.
As for the keel, if it's external, be especially wary of brokers or owners who try to laugh off a "smile" (sounds almost like fun when you call it that...) where the keel is pulling away from the hull.
Having said that, and given that we are talking about fiberglass boats, any
frp boat can be made better and much stronger with sufficient infusions of cash, epoxy, and skill. If you find the perfect boat for your dreams, you can make it into your dream boat.
However, it's always better to start off with a good "foundation".
Happy Hunting and Fair Winds!