"...never competed in a pistol match..." Go anyway. Both IDPA and USPSA are shooting games. The more experienced shooters will help you in any way they can. Shooters are just like that.
"...what mods if any are allowed or recommended..." Go to their respective sites and read the rules for each. Might as wel lread the IPSC rules as well.
Shot a stock 10mm Delta Elite at a pin shoot when they first came out, long ago. A Colt Rep was there(a very good bunch of guys. Met one at Second Chance too. He had an M16K. FA 9mm SMG verison. He didn't care that he came back to his table to find two guys loading his mags with our IVI ball.). Provided the pistol and ammo(Norma. There was nothing else then). The guy knew it wouldn't compare with gussied up pin guns(didn't with my ungussied up Series 70 Government either) and that the availability of ammo was low. Not bad considering the slightly heavier felt recoil. On par with hot .45's.