When it comes to dirt bikes, you basically have 2 main varieties; trail bikes, or motocross bikes.
The bikes listed above are trail bikes. If you rode something similar before, one of those will be fine.
But if you want to send it over the doubles, blitz the whoops, thrown down some gnarly whips, or clear LaRocco's Leap, you're gonna need a motocross bike.
Motocross bikes (YZ's, RM's, KX's, CR's, SX's, and that other one...) are full-on race bikes, and are very fast. You should probably have spent some time on a trail bike before you jump on one of these.
Whatever bike you choose, I would plan on it being a temporary bike. As your skills improve bike requirements typically increase as well. So spend accordingly.
Just my $.02.
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