Different organizations have different rules about what guns and modifications are allowed. Some have "major caliber" rules.
You might want to identify the organizations and matches that interest you, and check for links to their rules on their websites, before you choose gun and mods (if any).
Edit: as to the safety, I got rid of mine on the M&P that came with a thumb safety. I shoot IDPA, and the idea there is to shoot what you carry. If I liked thumb safeties on carry M&P pistols, then I would use one in IDPA; but I don't, so I don't.