[[[[.......I don't like the build or body language of the first mare.....]]]]]
What body language? She's been set up in a show stance and the photographer has whistled, or opened an umbrella, or squeaked a squeaky toy to get her to put her head up, her ears forward, and look alert.
I like the looks of the first mare better. The second one is soggy looking and the first just looks finer bred. The second looks like she could handle more weight.  Actually, I suspect that the first photo was taken when the mare was a lot younger. Maybe when she was being shown, because she sure looks young to me.
Some of the difference in size is fat.  The second mare is pork fat and the first is in working trim. There is some difference in bone but not quite as much difference in size as you'd think at first glance.
However, I would not buy any horse that I had not seen in person, run my own hands over the legs, tacked up and ridden by myself and before the owner took her out to work the buck out of her.
Personality is a huge factor when you buy a horse to ride.  You want a horse that you knick with.  You want a horse that is a joy to be around and that likes to be with you. It doesn't matter how experienced a horseman is, he can't tell you if the horse will suit your personality.
I can go and look and a horse or a dog will behave perfectly for me and not be the same animal at all for someone with less experience or a less alpha style or that has stronger hands or weaker legs or a different seat than I have.  I use my weight and my voice. A rider who relies upon their reins is going to get something different out of a horse than I do. So how can you expect someone else to evaluate a horse for you?
I once went out and looked at a superb horse. He was gorgeous and perfect under saddle. Then the person who was thinking of buying him, who had years and years of riding, got on him and was terrified.  He was much too powerful and too finely tuned for her. She was sure he was going to run away with her, and he very well might of if she rode him several times, because her riding style confused him. Yet I would have highly recommended him.  Gorgeous animal, a real Ferrari of a horse.