is it advisable to buy a property without original documents

Certified copy of the Registrar is sufficient provided it is genuine certified copy. In the mean time appoint a title investigator (Escrow)  an Advocate and obtain his report before finalizing the matter.

attach the certified copy to purchase deed.and counter check  the copies.

Issue public notice inviting claim or objection from general public that if anybody is in the possession of said documents and has any right or title over it( in case of equitable mortgage, documents may have been submitted as security) them and they be produced if found in possesion of somebody having a right over it. After that if still documents are not traceable then take indemnity of the vendor that if anybody claims in future he will indemnify you with all costs.

U further serch a 30 years records of Sub-Registrar office and revenue records office then u give a Public Notice and take a Title Clear Certificate and Title Report of the Property, and then Tilte is clear of this property u purchase a said Property. Certifide copy of the documents is velid when the original copy is missing.  

Certified copy has the same legal validity and sanctity as that of original.As suggested by others , better way will be to give paper notice in a widely circulated daily and national N Paper.If any one has any claimof whatsever nature will come forward. Even if such person doesnot lodge objection,you will be bona fide purchaser in the eyes of law.