Registering property through the builder's lawyer

Hi.  recently I am interested to purchase a preperty from one builder in bank loan. All formalities have been completed successfully and now registration date arrives. As advocate of builder initially provided all documents like (agreement paper, plan sanction paper and other bank related documents), that's why builder is forcing to complete the registration process through same advocate. Now first of all I want to know if it's safe or not? If answer is yes, then what are the documents I should check properly at a time of registration (Please list them). If now how much it would cost me to hire an advocate to verify those documents on my behalf. This flat is located at KOLKATA