It is always advisable to get all the relevant papers, documents of the property properly scrutinised by an experienced lawyer in the local before purchasing the property which involves huge investment and has the history of litigation in the past.
The piece of information you have given here would not be sufficient to render a proper opinion about the genuineness of the property.
The authorities will not grant OC if there is a deviation beyond the specification and if the authority decides to demolish the deviation of construction violating the approved plans, then you cannot claim any excuse at at that time.
'A' Khata cannot grant exemption for the deviation in the construction.
The high court order copy is to be perused for the details about the judgment confirming the ownership as claimed by you.
Purchasing the property without OC is not advisable.
The bank would grant loan now, but when there is a problem in the future, he would recover the loan amount mercilessly by his own means and sources as per law.