First of all you should be aware that neither the Will nor the sale agreement is a valid title document.
If the C is holding GPA, then his principal should be live o this date to ratify the sale deed to be executed by C in your favor. A gpa cannot execute GPA to another person in the name of selling the property, it is invalid in the eyes of law and the supreme court has come down heavily on this.
Obtain proper legal opinion before buying the property.