whichever one you join.
You have ample magazines. You could even use "large capacity" magazines in USPSA (IPSC) events, so long as you did not load more than 10 rounds in them.
A Kydex holster should work fine. Talk to people using SIGs at the matches and see what equipment seems to work well for them. It will at least point you in the right direction.
Here's the USPSA website:
http://www.uspsa.org/
Have fun!
