Ammo: For competition you choose ammo based on performance and price. Better to have 2000 rounds of ammo that cycles your pistol reliably than 1000 rounds that groups a half inch tighter that also cycles your pistol reliably.
Which brings up "handloading" and the reason that most serious competitors (with a few exceptions such as the wealthy or the sponsored) reload on a progressive. For making holes in targets cast/plated is often as good as FMJ.
Pistols: In terms of a factory firearm, reliability is more important than accuracy. Clearing a malfunction eats up more time than a miss that causes an extra mag change. Hence a lot of Glocks on the line. Between pistols that are equally reliable (a heated debate in its own right) pick the one that fits your hand better.
Gear placement: this is really personal preference but I like mags on my waist, upside down, facing to the rear on my weak hand hip. A drop leg holster sucks to run in (at least with a pistol in the holster) and I haven't seen any advantage to a drop leg over a belt holster in terms of draw time (but USPSA isn't my game).
Machine guns are awesome until you have to carry one.