you have gotten some great info so far. You will want to get as many mags as you can, unless you are shooting IDPA. Then 3 will be plenty. Almost any mag holster will work, I have some of fobus, and they work well for comp, and cheap
Glock worx triggers, wolff springs, and good sites are some of the most common mods for a Glock. I like www.lonewolfdist.com for glock parts.