Who the pistol was made for. Beretta makes both. The M9 is simply the U.S. military designation of the 92FS pistol. The military pistols are marked as such. Functionally they are the same.
Both are available on the commercial market. The M9 comes in a cardboard box instead of a plastic box and costs about $50 more, but as stated above, functionally, they are the exact same gun.
The FS is the most advanced model of the Beretta 92. I wouldn't recommend the early M9 pistols (which were basically Beretta 92F pistols) over the 92FS, but the later production M9 pistols are 92FS pistols. The main difference being the the actual M9 pistol production is supervised directly by the DoD.
Yes
Condition 2, does not apply to the M9 Service Pistol.
an m9 with a silencer and a desert eagle with an extended barrel
The M9 is a standard 9mm round. the M9 is actually a .355 caliber
9mm
One would not need to purchase an M9 pistol. One may want to purchase an M9 pistol so they can go target shooting with it. Target shooting is a hobby many enjoy.
125mm
Ejector
217 mm
140mm
SF = "Short Frame" The short frame was originally designed to compete in the U.S. military Joint Combat Pistol trials for a new .45 ACP pistol to replace the M9 pistol. This was achieved by a reduction in trigger reach and a reduction in heel depth.