Don't bother with that hyper you asked about, it will be more headaches than it is worth. Outside of all the full suspension bikes, there's only a handful of them that would be good on a pumptrack and those are either likely well over your budget or comes as frame only which gets expensive.

Zephyr11 is spot on with the bmx bike thing. Get a basic bmx bike, something from danscomp or a lbs that is within a doable range with your budget, then save your money as you get more experience on it then get a 24" or 26" of your choice.