Spare engine rebuild???

[DT_Duh_Trucker]

What is everyone's opinions on buying a used or rebuilt motor isx2350 to have a reputable shop rebuild as a spare or back up motor? If I have it put in my truck once done I could rebuild the original motor over time again for when it's time to swap them out. Was thinking could possibly cut down on down time far as the rebuild process not including the time to swap and everything. Is this a waste of time or money or am I not to far off from a good idea? My truck has 483k on it now. But I bought it with 457k. Haven't had any issues that make me think I need a rebuild yet but I'm usually the kind of guy who likes to be more on the always be prepared side of things.