Re: Used 6.0's w/ replaced engine?
Engine-wise I wouldn't worry about it, provided the engine was replaced under warranty. If it was replaced not under warranty, I might be concerned the engine had to be replaced because of a lack of proper maintenance. If you can get the VIN, and see if one of the Ford techs on some of the other forums can run an OASIS report for you, that should tell you why the engine was replaced.
I would be sure to make sure all the SMB installed options work, since the body was probably removed to swap the engine.
SMB-less as of 02/04/2012. Our savings account is richer, but our adventures are poorer.