HYPP is the result of "Impressive" inbreeding, that is, "Impressive" blood on both sides. I'd keep the horse, but don't knowingly buy another one.
That is not a true statement. HYPP is a genetic desease that traces soley back to Impressive. Impressive was the original carrier. He was at the time, probably considered the perfect specimen as far as what an AQHA horse would look like so many, many people bred to this stallion.
It took years for people to realize HYPP even existed and once they did, it took even more years to trace it back to Impressive. By then many many horses had been bred and were now carriers.
It had nothing to do with inbreeding.