Saggy belly means nothing. She's an older girl. She is a tad overweight, and you'll be able to get her a bit more trim, but she has the same shape as our 10 year old.
If she was used as a breeder's dog, she may be saggy because she had litter.
I wouldn't worry about the overweight. If you are going to adopt her, go for it. You can help her later on with diet and exercise.