You are correct, the Prairie is built like a tanks. The stringers are all glass as is the hull. Generally speaking, I have never any problems with the Volvo. My boat has the 4-108, which is a little under power. The 4-236 is, I believe just about right. Better to have more power then to little. I also know that many early Prairies were built wit Perkins 4-154's. This engine is very difficult to get parts for. I can not speak to the construction of the Atlantic version vs. the Prairie, perhaps someone else can address that issue.
The Prairie is a well built little vessel and handles well. The do roll in a beam sea and they are not fast. They are efficient and easy to handle in tight situations. As always hire a surveyor for the hull and another for the engine.
John