Whether he first gets elected in 1996 or 2000, what would happen if Buchanan was President when 9/11 happened? Would he fuel hatred of Muslims? How would he feel about Afghanistan, airport security, the PATRIOT Act, Iraq, etc.?
Honestly, the kind of butterflies it'd take to get Buchanan as president other than as a Designated Survivor cabinet member would make 9/11 either different or not happen. I think you'd need a Dukakis victory in 1988, then a non-Bush Republican win in 1992 that strongly pushes for scaling back the American international involvement with the end of the Cold War. Take it or leave as far as a 1996 election goes, but perhaps Buchanan is the VP in this Republican candidacy. Then he wins the 2000 race.
I think you'd see a much stronger constitutional push for explicit profiling/tracking/registration. I think you'd see major equal protection cases happen. There'd be an extremely violent overthrow of the Taliban with a "no-BS" occupation that's main purpose is to Japanize Afghanistan. Iraq wouldn't happen.