Recently I had a transaction and it includes the following papers representing a VALID sale and purchase of a vehicle:-
- Transfer Deed
- TO Form (Which used to be a simple piece of paper with some stuff printed but now it is on stamp paper)
- Sale Receipt (Remains with Buyer)
- Delivery Letter (Remains with Seller)
- CNIC of Seller and Buyer
- Registration book offcourse
All the expenses of above are from the pocket of the buyer 
Verify the reg, chasis and Engine No from CIA office for any mis use of the vehicle (involvment in acts of theft or murder etc etc)
Then for your comfort, verify the documents from the Registering Authority office otherwise when you transfer the car to your name, everything is checked
Just a piece of suggestion that please please don't waste your time by not getting the book transferred to your name - Just do it as soon as you are driving the vehicle because in the prevailing circumstances, we shouldn't be taking any risks - Hope it helps 