El Bull,
You can pretty much bet on the transom being gone or on it's way to mulch. It's a '79 - the vast majority rotted and fresh water is even worse than salt for rot. Every single hole in the transom of my 23 showed how the rot progresses and it didn't matter if epoxy or 5200 was used to seal holes; every hole was 'properly' sealed

- but they all were starting points for rot. Ditto on the deck but you can patch it piecemeal until you do it all. The 18' is a great hull though. It is definitely worth bring back. Your offer plan seems ok with all the work to do, issues on the trailer, and an unknown on the motor.