Benjy, I know you are new here and absolutely nothing wrong with your question. I think to answer it, we have to break the boat into all of it's parts. There are likely very few people that own that boat on here....BUT, there are many people that own Mainships. They are decent boats and represent great value. The generally use decent hardware. They would be synonymous with a Ford or a Chevy. The heart of that boat is those engines....Yanmar 4LHA-STP. I have owned one and they are pretty highly regarded. I put about 900 hours on mine before I sold it. 900 hours of completely trouble free operation and I would expect many more after that.
If you plan to cruise that thing below hull speed you might want to check out the "underloading" thread. There is a lot of myth and legend in underloading a diesel but there is a lot of good info on that thread...if nothing else, just food for thought. WIth that said, that engine does have a seawater cooled oil cooler. And it is a very good one. Meaning the oil remains too cool at low power settings. Will it hurt your engine to run at low power settings all the time???? I think you need to read up on it and do some research. I will tell you this....I could NEVER get my oil up to a decent temperature to change it. Idling in the slip did not do it. So I would run out into the lake and run it hard....but the idling done in the no wake zone back to the slip had the oil cooled off before I could change it. Anyway, all I am saying is that oil cooler works well and you may need strategies in place if you plan to run that boat at low speeds. This is absolutely not a "problem" with those engines. But they were designed to SCREAM!!!....and that is how I operated mine. 2800rpm cruise.
The Mainship 40 is a perfectly fine boat. If it suits the type of boating you plan to do, I am willing to bet it will serve you very well....and at a decent price point as well.