Documents i should ask for before purchasing a property on power of attorney

Hi, I am purchasing a property in an unauthorised colony on Power of Attorney and Agreement to Sell. What are the documents and affidavits that I should insist upon from the builder so that I make sure my investment is safe. Are there some documents that can be executed before the SDM / First Class Magistrate. What kind of receipts I can ask from Cash Payments and payments through cheque. Kindly guide.