With 45 ACP and 308 in your stable you could consider shooting heavy metal/heavy optics, if the club you shoot at offers that division in 2-gun.
Get yourself an inner/outer belt combo from
Shooters Connection.
They have holsters, mag carriers, etc. And they ship super fast.
I use bladetech mag carriers.
Don't go too crazy buying gear until you have a few matches under your belt. You can see what other guys are using and have a better handle on what you need.
Honestly a back pocket works great for most AR type mags.
Get a retention holster. There is too much dynamic movement, running, jumping. In and out of cars, snow fence.... etc. A hot gun on the ground is a DQ and a trip home. You won't move a place up or down because of the speed of your draw in 2 or 3-gun.
I think the Safariland GLS Pro-Fit is the best thing going right now for a competition holster with solid retention that doesn't impede the draw. I have an ALS as well. The nature of the thumb drive is detrimental towards nailing a really good grip.