I wouldn't be at all surprised if the owner was right and the horse found the whole process of being tried out overwhelming. At age 5 mine certainly wouldn't have coped with an unfamiliar rider hacking her and then taking her for a canter, or with multiple riders one after the other. I suspect the owner now realises that the horse needs to get comfortable being ridden by different people before being sold to or tried by anyone else.
I would not get a young horse if you want to compete straight away, and because you now know you don’t want to sit the silliness that comes with young horses. I would aim for age 6+.
On the cob debate - I have a cob x trotter who was mind blowingly inexpensive because she is full of absolutely endless energy. She’d need fully clipping at any time of year but would do endurance with enthusiasm and still be raring to go at the finish line!
Buy a horse that ticks that boxes when you view rather than having potential to be the right horse for you and your daughter. Just because potential takes so long.
As a family we have done lots of endurance and I competed on an irish cob. They take longer to fitten but are awesome. We have owned arabs too and my cob had to baby the young arab around his first rides. New forest ponies, connemaras etc are all great endurance ponies too.
Don't get caught up in a specific breed but go looking for a horse that suits you both. The main thing is that the horse is sound, has good confirmation and is comfortable to ride. I have seen all sorts of horses doing so well. Endurance is wonderful, it cements horse and rider partnerships with long hours in the saddle and you don't need a specific breed just a horse that you love.