Depends on the price really
I have a 2014 MN triton and have owned it from new. Absolutley bloody love it, and it has never let me down (apart from an injector pump which was a stuffup by the mechanic, which they covered). Taken me all over the place. I have 135k on it now
My brother bought an identical 2014 MN triton, and had the engine replaced at 90k.
So apart from everything that people have mentioned above, which is solid advice in buying any second hand vehicle, unfortunately when buying an MN, it really is a bit of a lucky dip. If you get a solid unit and for the right price, they really are wonderful cars, but you can also get a dud.
With all the engine stuff said, when my brother engine went, the process of getting a replacement was not that difficult, and it did not cost him a thing. There has been reports on this forum of engines with 200k+ being replaced, so you will no means be left high and dry if you are unfortunate enough to get a dud.