"Non-payment of consideration amount would not be sufficient ground for cancellation plus physical possession was not given?"
Such clauses are incorporated in Sale Agreement; not in Transfer/Sale DEED. Check for such clauses in the instrument!!
"Is registartion manadatory ? or mere paper ads by both parties would be enough?"
Without registration, a permanent defect in TITLE of the property would be created, which would be difficult to solve in future!!!
"what is the stamp duty & registration cost, if regisration is mandatory?"
First of all, the cancellation doesn't seem to be applicable. But if possible, not much as compared to the present cost of the property. May be only registration charges.