seller says stringers, transom and floor are rock solid
floor & transom should be an easy enough inspection, stringers kinda hard to tell until you really dig into it.
if it was built the same way an old aquasport was then it will have foam filled fiberglass stringers, they won't rot but can delaminate from the hull.
ballpark cost to replace a floor like this?
doing all the work yourself the cost isn't so bad, but finding the time can be tough. paying someone else to do it can get costly. many many other questions too, what type of core material for the deck? you should have about 20 sy of deck there, time to put together a spread sheet - glass, resin, core material, paint, tools, other, then add 20% to that cost.