@salus_products,he is not a seller and he is confused about buying that phone.......read the full question at first ! LOL
@dsamantray,this is the right place to ask the query 🙂
And you need not update the firmware as it will be updated the normal way.
Still,my suggestion to you is to not buy any phone which is under seller warranty. Most of them have defects in features and in case of any problem you will have spend extra bucks for sending them for repairing. Manufacturer Warranty will cost you some extra bucks,but that is always a reliable option for a buyer.