When I was shopping for a young WB prospect (focusing on 2 yo’s, so couldn’t ride them), I really focused on:
Temperament - will I actually be able to ride this horse? I researched stallions with reputations for throwing amateur-friendly kids.
Gaits - will I actually be able to ride this horse? I wanted something with gaits that were elastic, but not extravagant. I turned down a few horses that didn’t have the shoulder freedom, and a Weltmeyer daughter that I thought had too much trot (plus I was worried about the spiciness that might come with Weltmeyer)
Conformationally, I looked for a nice, longer hip with a stifle set down below the body (if the stifle is up hugged into the body, it will be harder to bend that joint to sit in collection). I also looked for good bone - no little feet and spindly skeletons for me. Decent neck with a clean enough throatlatch to maintain an upper-level frame without constricting too much. Strong loin connection and nice shoulder angle. But most of all, overall balance.
Movement - aside from gaits, what attracted me to the mare I ultimately purchased, was her ability to flow in and out of gaits, and natural tendency to step under and stop herself with her hind. I was impressed that in a free-lunging video, she didn’t invert and pronk around, but rather kept her hind legs engaged.
She has turned out to be a sensitive soul, and starting her hasn’t been the easiest, but in terms of ability, I am pretty happy with the horse I chose, and how she is maturing, as a 4 year old. I would still take her over one of the more modern-type, extravagant-gaited horses any day. If we can get her going in the right direction, I think she’ll make child’s play of lateral work, and hit the sweet spot of being able to both extend and collect, rather than being strong on one and weak on the other.