Now the property is a charge to bank. If want to buy the property then entered in to a sale agreement for buying the property from owner. All the details about the loan, sale amount, duration for execution of the sale deed etc..are clearly mention in the agreement.
The same time meet the bank officials and search the full liability for closing the loan. If you need cash for purchase the same then apply a purchase loan.
1) Should I buy from bank by paying full amount to bank? (later bank will pay off him after-debt amount)
You can buy the property only from owner after discharge the liability .
2) Should I buy from owner by paying him advance so he can pay off interest, release document?
You can entered in to agreement for sale to owner paying advance and try to settle the loan by approaching the bank with the help of owner