My rule was, if I am attending an event rarely in my life, then rental is more appropriate. But semi frequent, like one event a year, buy a black suit. It doesn’t hurt to have, and also a blue navy blazer for semi formal events were you have dress up but it is not tux formal. A black suit can be made very formal with white shirt, tie and loafers. Or more informal with fun shirts and more fashion shoes for other big events. A well tailored suit is a powerful item in the wordrobe.