For 500 with tack, if she passes a vet check, I would take her. If you got the trial, take her out on trails, where she might see unexpected scaries and see how she reacts. If she just blows and walks on, she's great. There are a lot of concerns that people will watch out for, but what if this is THE horse for you? I wouldn't let the weight issue hold you back. You can still ride her and she will pick up weight while you have her with the proper care. If it turns out that she may not be perfect, you can work out the probs, or find her the perfect home that will be WAY better than just selling her to anyone, you know? Just my opinion, hope everything works out!