Legal aspects related to an Assignment transaction

1. We (Party A) booked an apartment in a ready to move apartment complex (Nov 2021) by paying 10% of the property cost 2. Partial OC for the apartment complex was received in July 2021 (OC received for 17 floors out of 21) 3. The apartment is an Investor unit and we will be getting into an assignment transaction with the original purchaser (Party B) of the apartment 4. Party B happens to be a Partner of a partnership firm of the Builder (Party D) 5. The Principal agreement of the apartment was between the Party B and the land owners (Party C) 6. This apartment was part of the share allotted to the Party C by the builder (Party D) 7. This project looks clean (BBMP approval verified,EC verified, Khatha verified, Servey no. verified,betterment charge verified) and the builder is one of the renowned builders (CatA/CatB builder) in Bengaluru 8. This assignment transaction is being handled by the builder (since B is a partner firm of D) 9. Now, while entering into this assignment transaction, the new draft "Agreement of sale" mentions that B holds the general POA to carry out this assignment transaction on behalf of C 10. The GPA document is available, but it is on a Rs.200 stamp paper and notarized, however its not a Registered GPA 11. Now we need help with the following questions: a. Is a Registered GPA agreement mandatory in this case from legal perspective? b. With which party will we be doing the Registration of the apartment - B or C? c. Can there be any legal issues in future related to title of the apartment as the GPA is not registered? d. Any other legal aspect related to assignment cases which we need to be aware of before signing this sale agreement?