Fortunately, I am not in the market for a new engine (yet!), but it is one of those things I keep an eye on because it can come up suddenly when dealing with an aging powerplant. (7400 hours and counting)
I dislike the idea of a turbo for all the reasons listed by other people here, and some more too. I find the turbo charged engine to be very unresponsive in close quarters maneuvering. Power is low until the RPM builds up to engage the turbo which happens in an unpredictable amount of time after I throttle up--and always too late. In a strong head wind and steep chop the 75HP turbo has trouble getting enough rev's to generate the power it needs to get up the power curve far enough to drive the boat.
A Volvo D2-75 turbo engine weighs 582 pounds (264 kg).
And the Yanmar 4JH4 turbo weighs only 482 pounds.
Beta Marine sells a naturally aspirated 75 HP engine based on a Kubota block. A nice engine with a lot of good history. It has a nearly identical footprint to the Volvo and is even available with an isolated ground option. Looks like a very nice engine for a SM (And here is the "but", and it is a big one)... it weighs 911 pounds (414 kg)!!!
I'll put up with the complications of a turbo to avoid carrying an extra 450 pounds in my boat. I'll still whine unattractively about the turbo, but that is a LOT of weight. Unless somebody knows about a 600 pound naturally aspirated 75HP marine diesel.
Solomons Island, Maryland