If she was in a breeding environment for 7 years, I seriously doubt she will ever talk but who knows? They are very intelligent animals and they do feel the need and desire to communicate so there is always a chance.
Now, her diet will take time and effort but you can improve it - no doubt about that! I don't feed pellets because I don't think they are the healthiest dietary option for them, I feed gloop and have successfully transitioned -literally- hundreds of birds to it without a problem (not to my credit, mind you, it's that birds like gloop) so that's a possibility for you to consider.
Now, I am not trying to dissuade you from buying her, quite the contrary, I think it would be great if you did! but you need to take into consideration that the behavior you see in the store is not what you are going to get after what we call the 'honeymoon period is over. She is now scared and confused so she is now on her best behavior but this will change as she feels more comfortable in your house. Personally, I would take her straight from the store to an avian vet for a complete check up because a breeding bird that did not produce in 7 years and is so tame that she can be handled raises a red flag in my mind... I could be wrong (of course!) but, normally, breeding birds are not 'nice' to strange people (or even the ones they do know

).