For your second body I would consider used. Its your second body so does it matter if its slightly worm or has a few miles on the clock? - NO!
I would advise something better than your main camera if you are happy with D80. So a used D80 or possibly a D70/D70s as you will get a good camera for little money now D80 has been available for some time. Also consider the battery type so you can share batteries between the two, bodies can be different just try to keep battery and card type the same.
I have two bodies but do not often use simultaneously, after doing this for a while you may find its easier to just change a lens! However a second body is a great backup and if you second body has features/properties that the primary does not have then its a bonus. E.g. I use my D2Hs in darker conditions because of its better noise profile whereas D2X is used during brighter conditions.
Mirror slap! - I think its only relevant on a tripod where tripod mass and collar design are key factors. It does not occur with hand-holding as your body is sufficient mass to absorb any slap.
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