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)