I'm glad she is feeling better and you are getting her spayed.
Here is some great information on struvite bladder stones for you:
Canine Struvite Bladder Stones
As far as food goes, unless you plan on homecooking which would be much more expensive, she should stay on a prescription diet. Since Yorkies are so small, a bag will last you awhile.
Since she is seven now, she is officially a senior dog. It is normal that she will develop more health issues as she ages that will require vet visits, medications, special diets, etc. so be prepared.