Com-Pac usually installs a 6" screw-out deck plate in the cockpit, under the teak seat, to access the transom. I'd be surprised if the boat you're considering doesn't have one. I remounted my adjustable motor mount working through the deck plate. It wasn't easy, but it was doable. Others have removed the deck plate trim ring so they had a bigger hole to work through. I know of at least two owners who removed the horn that the tiller passes through. That gives great access, at additional work of course. If I were to do it again, I'd consider installing a larger deck plate so future access was easier. Lots going on back there.
The transom is cored with plywood, Com-Pac just used washers in the installation of my ladder, rudder, and motor mount. I've had no problems, and I haven't heard of any failures. Bed any fasteners properly, of course.