Cruise,
Two years ago, I purchased a 1984 MK III, that had been repowered with a cummins bt 210, had a bow thruster (which I HIGHLY recommend on this boat) 5.5kw genny, and NO soft spots. Of all the Mainships of that vintage, IMHO the MK III is by far the nicest, with the transom door, no bulkhead between galley and salon, separate shower and ratio between cockpit and salon. I paid in the neighborhood of 30k, a bit more than some Mainships I looked at but was in good condition and HAD BEEN repowered, which for me was an absolute. I had looked at quite a few with the original perkins, and after talking to some of the mechanics at the marinas, warned me that some vintage perkins parts are extremely hard to obtain, as an example, the turbos and manifolds. I did spend some time and money upgrading the interior and there will be some mechanical upgrades I will need in the future, but on the whole it's a great boat. MY 2 cents is to find a repowered MKIII with a bow thruster that is in decent shape. It won't be the cheapest MS that you find, but in the long run will give you the most pleasure and the least grief....