What to look for when buying?

For checking out a transom, I've always been advised as reelapeelin stated, with the engine tilted up, exert a lot of force on the bottom (prop end) both up and down, and look for flexing or any movement in the transom. There should be none if the transom is good.

As for cockpit floors, just walk around on them; they should feel solid . If you detect soft spots, they are in need of repair.