To purchase property from a gpa holder

I have paid money to a property holder Y who does not have a registered sale deed , instead has notarised GPA from his previous owner X dating 1 January 2009, a sale agreement and a affidavit promising the authenticity of both GPA and sale agreement. On basis of these documents Y has got municipal e khata in his name. The khata PID shows Y's name now. The property can also get registered here. But there is absence of sale deed between X AND Y. One more problem is that X has expired in 2019. my questions are 1.Is it legal to purchase this property on basis of the notarized GPA in which he had given powers to sell the property is that sufficient as it is a notarized GPA dating 2009 before SC ORDER OF 2011. 2. Is it valid if I get the legal heirs of X as consenting witness in my sale deed. 3. If the legal heirs of X deny to witness as consenting witness can my vendor Y FILE A SUIT FOR SPECIFIC PERFORMANCE on legal heirs of X. 4. X had converted land but did not form layout from land development authority nor Y has obtained license for house construction. Will this points be observed by judge while filing suit for specific performance, but municipality has regularized the building and property and has granted e khata. a affidavit enough from the legal heirs of X that it is in their knowledge about registration and they acknowledge it, as a local lawyer suggested me about this.Is this enough or sufficient above all. THE NOTARIZED GPA , SALE AGREEMENT, AND AFFIDAVIT ARE DATED 1-01-2009. I am paying the fully stamp duty now for registration. Will this help that complete stamp duty has been paid.