@Poster
The open letter is a Stamp Paper which is signed by the seller and the buyer n used to transfer the car to buyer's name. These days transfer rules r bit tough. Previously u just need the documents to transfer the car to your name but now the Seller and the buyer noth has to b present in order to transfer it. So u must b clear that the car is on the name of person who is selling it otherwise there might b a problem for u.
Documents can b checked from Fareed Court house also there is a website on which car information can b obtained i dont rem it right now. Beside checking the documents do check the car for any involvement in Criminal activities this can b checked through CIA i guess from the same place the documents will b checked.
Most important take your reliable mechanic wid u who cn check the ride for you so dat u dont get deceived.