I think the break-in is the main factor. Done right, and it'll last barring any mechanical defects. Do it wrong, and your engine could go under 1k-2K miles.
Completely agree. When I broke mine in I was diligent to not just follow the factory recommendation but also use some knowledge gained as a young man learning as much about cars as I could. That meant changing the oil early and often during break in to remove any particulate matter that may have been a result of manufacturing. Extra expense, yes and cost too, but never regretted the piece of mind. Got flamed by some on this forum for it but it wasn’t their money or their engine and though I’m far from 100K, the 37,xxx I have has been problem free.
I wouldn't buy at that price either, but I would be surprised if most of the miles aren't super easy highway and city miles. The engine is probably in better condition than many of the "collector" engines with 3,000 miles on them that sit with dry seals and rust inside the bores.
It has definitely proven itself. One of my first engine rebuilding instructors way way way back in the day said either an engine will fail immediately, or live a long life (as a result of the building process itself and assuming proper maintenance). Very few will fall in the middle.
That's the "bathtub curve", a well known statistical characteristic of the failure rate across a product line that's in volume production. Early on, there are "infant failures" caused by defective parts or bad assembly that form the early side of the bathtub. In the middle, there's the bottom of the bathtub - a long stretch where defective parts have all failed and the surviving good parts haven't worn out yet. Then comes the other side of the bathtub - "end of life" as parts wear out and the equipment's performance falls below acceptable levels.
