I have been shooting a RIA TAC 9mm in ESP for a few years. I am not any blazing ball of speed so take my thoughts for what they are worth.
The only thing I have changed are grips (Ergo) and a magwell (Wilson). I am good with black sights and do not want the controls any larger.
I suggest you go with some kind of kydex/plastic holster. I like my Bladetech.
I have Wilson, Tripp, and Checkmate mags. While they all run reliably for me, I prefer the Tripp. It seems to load easier and the base pad is easier for me to orient. Buy at least 4 of the same capacity. You are not allowed to mix 9 round mags with 10 round mags. You can only use 3 in a stage but it is good to have a spare and it also gives you a "Barney" mag to chamber a round at the start.
If I was you, I would shoot 5 matches before changing anything except maybe the magwell.