The first thing you want to look for is natural movement and conformation. You want a horse with a short back, well built hindquarters, muscular topline, naturally level neck, thin neck, and an attractive head.
You want a flat kneed gait. What that means is, when the horse jogs or lopes, you want as little knee action as possible. The legs should swing out as straight as possible.
The next thing to look for is natural ability. On the lungeline, without contraptions on, can the horse jog slow? Can it lope slow and rounded? A well-bred pleasure horse should be able to go naturally slow and balanced.
Breeding comes into a huge part of my western pleasure horse. I want bloodlines in the immediate dam or sire side. You'll have to do some research to see which bloodlines are "trendy" right now. I only know APHA.
And finally, you don't want a horse that's been "shut down" terribly. Personally, I don't want a horse who has been rode to death in draw-reins. Stay away from the four-beat lope unless you are willing to train it out of them. Always do a vet check to make sure the horse is actually sound.
As far as the test ride goes, I want the horse to be incredibly supple. If you're going for an expensive, upper-level horse, he needs to drive from behind, and go on a loose rein. I want to be able to put those hindquarters wherever I feel like it - and definitely two-track. No four-beating. The horse should carry himself very collected. Spur-stop training is based on personal preference. I don't do it, but some people like it and that's fine too. Show experience, whether he has won anything, etc.