a friend of mine pointed out, horse must have a good comfortable trot. Trotting is an efficient gate and you'll be doing it alot. I've heard tell some horses have a pretty smooth easy trot. I wouldnt know. The ones I have been on would jar your fillings loose. Sorta why I am on walkers, which fall under doing endurance with a horse I have as opposed to shopping for one. Seriousness in the sport, what distances, what type of rides can play a part in your selection.
But a horse that can be a 100 miler winner, in hot weather, in rough terrain, as long as its sane enough to stay on, will also do well for a fair weather turtle rider on easy LD's.
If I was specifically shopping for an endurance horse I wouldnt sell myself short and get a horse that can be a long term contender.
Be aware ride results for horse and rider are available online, lots of readign between the lines can be found looking at ride results. So far most of the endurance horse adds I see with claimed AERC points tend to be grossly exagerated. Horses with multiple pulls or turtle horses advertised as great. Horses owned by people with years of top 10 finnishes, change horses and then start coming in back of the pack, and now hors eis for sale. As with everything buyer beware. Problem is endurance isnt a huge sport and you are just as likely to get a great horse that has never been exposed to the sport. Not sure what part of the country you are from but in VA, I have seen lots of good prospects being used as slow trail horses for sale cheap, but as Arabians tend to run small finding the bigger ones is either hard or way more expensive. I know a girl with a really nice 14.1 or 2 arabian morgan cross Id be all over if I was 100lbs.
So size appropriate, 7 ish yo, nice thick legs, straight back, high tail, big sturdy feet, ( get a PPE) sane, comfortable trot, Polish, Crabbit, Russian Arabian or arabian/ liteboody cross.