If I was buying an arab to do endurance I would want one that was already proven to be able to deal with the excitement of it.
I do love the Morgans, they remind me of the welsh x Tb that we have a lot of in the UK but they can also be a bit too high geared for anything that gets them excited so you have to be the sort of rider that can stay calm and relaxed and then they will give you 100%
Don't rush into anything and give any horse a really good try out and have it vetted for anything that might cause problems for an endurance horse that might not bother a trail horse that's going to do much less work at its own pace
An older horse isn't a massive problem if you aren't going to push it too hard and its had no wear & tear but I would prefer something around 8 years old that's joints are developed and it has seen the world outside of a menage enough to be able to deal with things
Hope you find something. Good luck