Amateur New to "Action" Photography

[BlueQuark1]

Hi All! This is my first post on the forum! I'm a physicist who's hobby is astrophotography. Living in NJ, my real hobby is...hunting ideal shooting locations that minimize light pollution.

I also have children. They're now in high school and I want to be able to shoot them live (in action) from the sidelines/stands. Marching band, swimming, soccer. I know NOTHING about this. I have a Nikon Z9 that I programmed for astro shots, but I know nothing about live action. My ask: I'd like to grab pictures of my swimmers (indoors, lower light, about 25m from the action) and also my marching band players (outdoors, 5pm lighting, slower than swimmers, roughly 50m from the action). I'd like 1 lens to allow me to do all of this. At this stage, totally thinking a used zoom lens or 400mm prime. Can y'all recommend something? Also, any reason why I shouldn't look for used lenses? The cost of new ones isn't worth it for me...

Thanks!!!!