Yes, 1/4 inch is all you need. Call the factory with the HIN number and see if they stringers are wood cored or fiberglass with foam core. If it is wood cored, try to get as close to the transom as you can. If the owner is not OK letting you take core samples, tell him to keep his junk and move on. I was helping a friend buying a boat few months ago and I was talking to a the seller (a woman) about inspecting the boat and she said "I know my boat is fine so I will not waste my time and my brother's time with any inspection" I told her "well then, I will not waste my time talking to you" She still did not sell it. We found a much better boat and the owner was absolutely fine with us inspecting his boat with core samples, he did not even attend the inspection.
You got your money in your pocket, you set your own terms. You are the buyer, this is NO sellers market.
The owner's answers for any questions is worth nothing. Absolutely zero value. It is not even worth the breath I will use to ask the question(s)
The boat is right in the front of you, answer your own questions by careful inspection. Double decking is not hard to find. Water in the oils, compression. Water test. Leaks. It is your money, you set the terms.
After you find out everything you need to know, you can start asking him the questions not to know the answers but rather to verify them. Basically find out of he is a laying SOB or an honest person who is telling you what he knows. Then you start negotiating.