There is nothing wrong with an NH horse (my mare is) as long as you know what to feed, what to look out for as symptoms of an attack and what to do when a horse is going into an attack or is having an attack.
UCDavis can test for the disease. It's $50 and you have to send in a hair sample (from the mane, depending on age) and within a week or so, you'll have the results. http://bringinglighttohypp.org/
this is a very good website that has a ton of informationi about Hypp.