IF you want a 0-mile motor, in this case, I would build it.
If you can find a used motor in good condition, I would go that route first.
Depends a lot on the situation and motor you are talking about. Since you are going AMC360, I would want to build that, with the typical upgrades to the oiling system, and a few fixes particular to the AMC V8s.
If it was something like a Chevy 350, I'd maybe think about one of the cheap crate motors out there that are a good value.
The LS engines, I love them, but they aren't plug and play by any means. I am building up a 500+ HP LS2 for one of my CJ's right now.