everything
Seriously though, you need to look for delamination areas all over, rotted transom, stringer rot, bulkhead rot, soft spots in deck, gas tank condition, etc.
Is the transom built differently than a I/o or outboard set up being a straight inboard? I’ll have to assume any wood will be bad. Seems to be very heavy fiberglass hull.
In theory, a transom for an i/o or outboard would need to be stronger than needed for an inboard. I have no idea if the hulls were actually built different. Was there an outboard option for that model?
Since it is an inboard, you won't have the typical "Grady bang cap" problem where water leaks down into the transom like you would with an outboard. However, that doesn't eliminate problems with the transom core. It can still be compromised from things that have been screwed into it.
How does the floor feel walking around?
In the end, all the research in the world isn't going to help. You won't really know what you're up against until you start checking it out. And... whatever softness/rot you find by waling around/tapping/sounding... assume the ACTUAL rot is at least 3 times bigger.
Here's what I did to my boat, which was originally a sterndrive (essentially the same thing as yours in terms of your questions)
Thanks. Going to just work on mechanicals, make a plan and start pulling apart this fall for my winter project. Maybe a good video for my son to put together.
But... starting on the mechanicals is putting the cart before the horse. If the hull/transom/deck/stringers/bulkheads aren't solid... it doesn't matter how good the mechanicals are as it's unsafe to put the boat on the water.
The only water this boat will see is a pressure washer. Want to look at onboard systems, water, head, tanks, lighting…see what we need to replace or restore.