anyhow, another thought I would say is I would almost rather prefer a good "all around" horse that has the best genetics available opposed to a highly specialized horse that's potentially inbred for generations and generations.
And perfectly understandable! It's hard to beat a good allround horse.
However - I don't know about the US but our brumbies here are nowhere near best genetics - Because no one is monitoring their breeding they inbreed, they breed bad to bad and often magnify undesirable traits. You might get one in twenty brumbies that are buitl anywhere near as well as your average decently bred competition horse.
Breeding for competition horses is a bit different betweent he two countries I think, as well. At least in my breed there is little to no in-breeding or line-breeding. There are generally only 5-10 tap root or foundation sires that might appear on papers more than once - And generally once on the top and once on the bottom, way back.
So generally, buying a purpose bred horse is a better chance at good genetics than a brumby/wild horse.