I know. I am not against spending a few extra dollars but that rebuilt one gave me sticker shock.
About a year or so ago, a guy online was talking about "rebuilt" 5.7 engines for $2500 installed. I called him out on it, telling him that you couldn't even do a quality rebuild for that price when you factor in parts and machine work, let alone installation.
I have never bought a "crate" engine. I have rebuilt every one since 1990.
A used 6.1 could be a good idea. I'd be tempted to tear it down to hone the cylinders and replace the rings and bearings.