Yes, If the car is On your name you dont have to do any thing.
If it is not on your name you should have authority letter from the owner.
If tracker is installed you will have to inform them about your trip thats it(and this has nothing to do with bank its tracking companies policy).
Other thing is that If you are trying to buy a car in Karachi which is leased and you are just doing agreement with seller. then that is illegal.
Proper way is owner pays off the bank and you pay the owner and get car transfered at your name. this is the legal way.
Second thing is that you can still do that but only if you trust the seller.
other thing is driving a Sindhi registered car is Islamabad( if its a big car like Land cruiser or some thing No problem then) other wise if it a corolla civic you will be subject to more checks on the police pickets.