If u choose to do the response of this card and attach it to another stark character, does the new character receive the added strength of 3 and military/power challenge abilities? I am guessing no. Also, do u have to pay the 3 gold if you do the response and play it from your hand?

As an attachment all Banner for the North does is give the attached character Stalwart. You don't have to pay gold to trigger its response effect from play nor from your hand (it would say so after "Response:" if you did).