There are thousands of vehicles out there on road with duplicate files & duplicate cards.
There is absolutely no issue as far as original police document is available for file lost.
Bring the duplicate file in Lahore excise and ask them to re-confirm. Or you can ask the agent to double check it for you. Will take hardly 500-1000 for getting it checked from excise office.
And if vehicle’s duplicate file documents are OK, then I will honestly suggest to DON’T BUY IT ON INSTALLMENTS.
Since you are getting good price, then buy it on full payment. He will further less 50K or 100K from the price on cash payment.
P.S. This model is fuel guzzler. It gives 7-8 km/l average in the city. And its re-sale would be utmost difficult in future too.