As a matter of fact you have collected almost all the vital documents except for the most essential document.
The sale deed duly executed by the previous owner to the present owner (i.e., your vendor) and also the registered sale deed document in the name of the original owner executed by the builder or the original owner whatever the circumstances may be at that time.
Without a sale deed on his name how will the vendor execute a sale ded on your name and without a sale deed on your name how will you be able to dispose the property in future i.e, for even making a gift deed or settlement deed or for pledging the property as collateral security for a loan, you need to have a title document in your name. If your vendor executes a registered sale deed on your name, he should have a valid and marketable title to the property vide a registered sale deed on his name.
Therefore realise the importance of the sale deed and decide wisely on all such further issues.