You need to follow up with Islamabad vehicle registration authority for the extra fee/penalty they might charge for late registration.
As for Sindh MVA (Motor Vehicle Authority) there is a additional paper work for late registration.
Buying an unregistered car, purchased a year back no issues if the owner is willing to hand over the car with all registration assistance, full payment of the vehicle not to be handed until vehicle registered and verified under your name.
It will be more better if you keep a Lawyer informed along with all the paper work and process.
All the best.