Hard to tell from the pictures if her shoulder looks "odd"
I think she's quite a beautiful mare and she actually might make a really good barrel horse. She has a nice butt, a sweet face, short back, but yes, she's got a belly to her. Since your going to be working with her, I'm sure you could whip her into shape.
She looks like she has a nicely shaped topline, her neck seems a little thick but I honestly don't see anything that says that she's not an attractive horse.
I'd still get a pre-purchase exams. Just to make sure she is sound. How has she been when you've handled her? Curious and willing or does she seems slightly stubborn?
"The greatest pleasure of a dog is that you may make a fool of yourself with
him and not only will he not scold you, but he will make a fool of himself too."