A horse flinging itself into walls injuring itself is not normal behavior, IMO. Pacing, screaming, pawing and rearing, kicking maybe. But, not totally self destructive behavior. I would be worried about turning her out unless you have a stallion pen with high and safe fencing. This behavior may not improve outside.
I agree with a trainer and DO worry that she may have been "calmed" by the previous owners to make a sale. Tread carefully as there really are dangerous horses. Do not put yourself in harms way, if you can help it.
I would consult the previous owners but really doubt you will get much help there, even if the horse has a bad history. It will likely go like this....."she was always SO well behaved here" "What have YOU done to her?" Even if this horse had crippled three of her previous trainers.
Be very careful and approach this situation realistically and with your eyes wide open. I don't mean to scare you, but I don't want you to get harmed.