I would have a farrier look at her first. I would think most injuries due to overwork tend to be on the front hooves, as these bear most of the weight.
Am I right?
A farrier would be able to tell you if the problem is treatable.
When looking to buy a horse, you must try to use your head before your heart.
If this horse is limited to walk and rarely trot, it won't be long before you are ready to progress past that, and therefore she will hold you back.
Unless you have the means to 'rescue' her as well as having a sound healthy horse to ride, I would pass.
You want a horse that you can ride.