My suggestions are that it has a nice neck, built uphill, strong shoulder, legs straight, elbow equal to their stifle (or close to it), clean ankles, and obviously soundness. If you want more information, I recommend Elisa Wallace's channel on Youtube. Here's a link to check out on Youtube where she explains what she looks for when buying a horse:
You want a level head horse for starters! You need to know who you are buying from, because most sell off they’re under achievers or horses with long medical history. Look at the confirmation, want a bit more build in the back then the front but from the whole frame-balanced.
I've heard that the long kind of leggy ones do good because they cover ground easily. Also a superstition that big ears means that the horse is bold, brave and honest