She looks good. I will say that I have learned that "ridden by beginners and children" may mean that no demands have ever really been made of her. They can be a bit spoiled. Most times they do not make a horse do anything it doesn't want to. So, when you go try her, I would intentionally be a bit demanding. For example-ask for transitions and expect them now-not when she gets around to it. I sometimes try to get them a little annoyed to see how they react. Then, just make sure the reaction/attitude is one you can handle. (ie no rearing, bucking, etc.) Flail around in the saddle and look like a fool, but you want to see how she handles you off balance.
Good luck-she looks like she could use to get some excercise! She is not ribby for sure!