I bought a used '87 seafarer 4 years ago and hired a surveyor during the process. There was moisture in the transom, but the price of the boat was basically the price of the 4-stroke motor on the hull with low hours as well.
There were no soft spots in the transom, so I decided to purchase it anyway.
I rented a portable lift to detach and hold up the motor so I could drill then re epoxied/sealed all the mounting bracket holes in the transom with 4200 sealant.
When the engine first came off, alot of saltwater and sand gushed out of the space between the engine bracket and boat.
I guess I am lucky, 4 years later the transom still has moisture in it, but the wood is still very hard to drill through, and there are no soft spots.
Maybe in my case the saltwater pickled the wood over the years.
Good luck, but as Dennis advised, a surveyor is well worth it IMHO.
Lee