Older Beretta 92s will have a straight dustcover, no radius cut in the grip, and will have no warning label on the dustcover. They will usually always have metal parts (trigger, guide rod, decock/safety levers, magazine release).
Newer Beretta 92s will have a slanted dustcover, a radius cut in the grip, and will have a warning label on the dustcover. The slanted dustcover frame is slightly stronger since it is technically a Beretta 96 frame. They will usually always have plastic parts (trigger, guide rod, decock/safety levers, magazine release).
M9s will always have the straight dustcover, and no radius cut. I'm not sure about warning labels.
No real difference between Italian or American 92s except for markings (M9 will always be made in the USA)
It really boils down to personal preference. I prefer newer Berettas because they have the radius cut which makes its slightly easier to reach the trigger.
I'd buy the cheapest NIB Beretta 92FS or M9 you can find.