Hi,
Inlines or V-6's?...just curious really.
Going out on the water in a boat is a crapshoot even with new engine's. NEW stands for
Never Ever Worked.
I can't answer your questions but I will say that success or failure for your plan likely will reside in the quality of care the engine's have received thus far.
If the vessel comes with carefully detailed log books that include maintenance entries that would be a very encouraging sign.
You might want to think of the boat as a highway truck or bus for determining the probability of making your goal.
What I mean is that a rough formula for converting engine hours to miles on an odometer is times 60.
60 X 5,000 = 300,000 miles.
The 71 series Detroit engine can easily reach the 500,000 highway miles in trucks and many have surpassed 1,000,000 miles before overhaul.
Having said that, you have to consider that marine duty is typically much more stressful on engines as it's often said a displacement hulled boat is pretty much always pushing "uphill". So, I would shave off a bit of longevity due to that.
Shave off a little more for the turbos being used to sqeeze out additional horsepower. Although, bypass blowers helps alot with that.
The turbos don't always make the full trip that the base engine will as they are hard working but delicate.
The same goes for injectors. Especially if the fuel isn't of high quality and kept meticulously clean and water free.
But, with good history and perhaps some pre-purchase inspection to include passing oil sample tests, I would say that you're well within the window of making it.
The 71 is a very tough, reliable engine that is famously made to run all day, night and week at maximum governed speed if properly fed and cared for.
Whatever you choose I wish you fair weather and good fortune.
