I've seen a few threads here saying the transom felt solid but once they dug into it they found it was soaked so planning to replace it is probably a safe bet. That bracket looks great. It's always nice to have access to nice toys.
I am going to eventually install trim tabs so I left plenty of room for them to be mounted.
You may wish to mount the platform flush with the original cut out. Would also add a little extra mounting height adjustment to the motor as well as tab actuator space though I believe they make shorter ones for a tight fit.
Boarding (I'm guessing after a swim/dive) is best handled with a ladder. My B Bracket platform had a folding ladder that stowed away under the platform on the port side. Because of the angle that it folded down to in the water, it was (and still is) the easiest boarding ladder I've ever used. To deploy the ladder you reached under the platform and released a catch, let the ladder slide out parallel to the platform and then unfolded. The angle formed was like a stair rather than perpendicular to the platform. I miss that ladder (and the bracket).Good luck.