I looked at Yanmar, Izuzu, Beta, and Nanni engines in the 50 hp range. Best availability for parts was proabably Izuzu, as Izuzu trucks are everywhere. but I ran into a boat in Cyprus whose Izuzu had broken a timing belt, which opened my eyes to the fact that even some diesels use timing belts. The Yanmar is the only engine designed for marine use, which means that there is not a big spare parts infrastucture for trucks/tractors/compressors/refrigeration/etc. There are more Yamars installed in boats than other brands, but it seems like I have heard too many reports of major Yanmar failures at low hours to be comfortable with that choice.
The Kubota engines are popular in tractors and small equipment (forklifts/bobcats), and one of my friends with a trucking company had his mechanics tear down and rebuild a 30 hp Kubota which had been submerged. The mechanics were very impressed with the engine design--there were roller bearings in places that they did not expect to see them in engines of less than 500 hp. The choice between Beta and Nanni is more a function of what language you can call the factory in.
So I ended up with a Beta 50 (hp at 2800 rpm) in my 30,000 lb boat. Its now 2 years old and has 500 hours on it with the only problem being the built-in oil change pump, and Beta fedexed a replacement to me in Trindidad at no charge.
For extended cruising we put about 350 hours a year on our engine, to supplement the solar/wind battery charging, moving in/out of anchorages, and motoring in areas of very light winds. As a result, we never use more than about 20 hp of the 50hp available, and a turbo would be more of a liability than an asset.