What all documents should we look for while buying a Flat?

All documents appear to be ok but please confirm if there is no restriction inposed by HUDA on transfer to the third party by the original allottee, as you do not appear to be the original allottee. This practice exists in Housing board colony flats.

If there is no such condition, HUDA will issue a new allotment letter in your name by treating this as a transfer case, charging the transfer fees. As no conveyance deed has been executed by HUDA till date in the name of the original allotee, the conveyance deed will be executed by HUDA directly in your name.

The documents like 'Will' or 'GPA' are useless as sale of property by GPA has been banned by the Supreme Court of India  in case titled Suraj Lamps vs State of Haryana.