Dont listen to Sunny, listen to Jay...if you listen to sunny you would not be able to purchase this car (I assume the seller doesnt have the payoff amount in his checking to pay the lender, get the title then sell it to you!)
With a bill of Sale- AND proof that you've paid the leinholder, you are good to go. The Bill of Sale will release his interest in the car- AND you've establihsed the current lendor will release their interest in the title upon reecipt of $xxx. You could also put something in the BOS whereby he indemnifies you for any legal expenses to get the title should there be extenuating circumstances- but Ive never had an issue.
Keep it as simple as possible...but no simpler.