Rebuilt trucks look pretty, but you have to remember the caliber of guys that actually rebuilt them (hate to put it that way, but it is normally the ones that were not able to do much else), I am sure there are some guys that work on the trucks that know a ton about these trucks, but on numerous occasions when I go to pick up trucks I feel like I know more about them than the 'mechanic' that has come out to jump them off.
I have also heard differing stories on what is replaced in a rebuild, unless the serial tag on the engine is run by Cummins and confirmed it was bought in 20xx, I would not assume it was replaced. I have 'heard' that a rebuilt truck is torn apart top to bottom, but I have also 'heard' that they only fix what needs to be fixed (visually), maybe change tires, filters, insulation, wiring harnesses, etc. The one that I bought that had been 'rebuilt' had more rust underneath the fenders and such than my non-rebuilt M925, that is my main reason for thinking they only replace what they HAVE to to make them look pretty, if it was torn down completely to the frame, the paint work should have been much better on areas that don't 'show' like under the fenders. After I sold that truck the air compressor went out on it, plus it had a pretty good leak from the transfer case. So, I had quite a few problems out of a rebuilt truck I had bought, and have had just a few very minor problems out of my M925a2 that I have put over 2,000 miles on (couple small leaks, and the hydraulic valve that operates winch needs to be rebuilt).
Often times the trucks that haven't been touched are the ones you want is what I have come to find out. That said, rebuilt trucks are easier to resell because those that don't 'know' just think that rebuilt means better. This is not to say all rebuilds are bad, the few problems I had on the one I bought were still not things that would 'kill' the truck or things that could not be repaired, just not problems that you would expect from something that is supposed to be 'new', which is why I think they only replace what they have to.