MVP: 1500 hours is not too much for a properly maintained engine, you're smart to get an engine survey. Ask for the maintenance logbooks, and during the survey they will send out oil samples for analysis; these are very helpful. We have a Cummins QSC so I can't speak directly about a QSM, though I think Cummins makes some great products.
NTs are great boats and we went out in a 42 last year, they are sister ships to American tug. they were in our top 2 or 3 when we were shopping but ultimately went with an AT that became available in great shape. We too have high hp and from personal experience, we really appreciate being able to push speed to the high teens once in a great while but we also mostly go about 8 kts and sip about 3GPH. When at 8 kts, our engine is very low noise and smooth.
If you haven't yet been on a sea trial; this is a great experience and time to get a feel for that boat; the surveyor should even let you take the wheel for a moment and he will try to take the boat to WOT. When on the sea trial, be sure to walk around and spend a little time not only in pilot house but also saloon.
iPhone and likely others have a free APP to measure peak noise levels, i've used this during sea trials to compare noise on different boats. Of course, if the surveyor has an engine room hatch open, the noise measurement won't be realistic
One other thing on NTs: some had a minor separation at the seam where the back of the pilot house meets the upper deck of the saloon roof; ask the surveyor about this but he will be checking moisture anyway. Even if it has this issue I believe it is a minor thing to seal/repair.
Best of luck to you and please let us know how it goes... and of course everyone on this forum loves to see boat pics!