I would focus on the 9mm; ammo is cheaper, recoil is less, and everything is scored Minor in Production anyhow.
An IDPA compliant holster will work for IPSC/USPSA.
Blade-Tech sells an outfit with a separate drop loop so you can wear the holster high-ride for IDPA and drop it down for IPSC.
I use Ready Tac because I shoot with the maker.
Comp Tac has a national reputation. There are others but Kydex pretty well rules the roost as a holster material.
It is hard to have too many magazines, but as a practical matter, you must have at least three for IDPA. You won't have to fire more than 18 shots in any one timed string so two is usually enough but the occasional downloaded start or mandated tactical reload will put you into the third. As will a malfunction.
Three is all you may use at one time; one in the gun and two on the belt. More is nice so you can move directly from one stage to the next without having to reload magazines in a hurry. I have six for my PM&P SSP, more for 1911s.
IPSC/USPSA does more shooting. I'd want five; one in the gun and four reloads.
Obviously you need sufficient magazine pouches. Many of us prefer singles so we can space them on our belt the way we want them. All the holster makers furnish magazine carriers.
You will need a good stout belt. IPSC allows the double belts where you can put your holster and mag carriers on the outer belt and leave them in position. It sticks to the inner belt with Velcro. IDPA does not allow that, your holster must be on a belt through pants belt loops. The Wildnerness Instructor's belt or similar heavy nylon belt as wide as will fit your pants is popular.
