how many miles is too much?

[MidLifer]

I've been considering a LS swap for my 71 for a while now, and have been looking for engines. I run across a lot with 120-140-160K miles on them, and am wondering if a 5.3 LS with that kind of mileage (iron block version from 99-06) is something that is going to need a teardown and rebuild or will it give another 100K+ of service before needing to be rebuilt.

The only anecdote I have is my neighbor's 02 Burb, which at 270K seem to have a stuck lifter as is getting noisy.

Thoughts from the experts? Also where are you guys finding 10 year old engines with such low mileage on them? In these parts they all seem to be getting driven.