Hi. I had a transom professionally replaced a few years ago on my 1975 Stejcraft runabout. the timber that went into the transom is much thicker and stronger than the timber that came out of it, so from a strength perspective, it is stronger now than it was when the boat was built. I have no regrets getting the transom replaced.
I agree though with Obey1 re the floor and stringers because it's now time for me to do the floor in my boat as it is "soft" in places, ie it has some riot in it. Old fibreglass boats are a bit like old cars, ie they need to be maintained and have money spent on them. If you like the boat you should buy it, however you need to know that it may require further work/money in the future. If you ever have to do the floor and the stringers, then you basically have a new old boat.
All the best with it.