Lucky you. You will love it so much.
Credit card can be used everywhere, even in public toilets, yes you have to pay for it

I did exchanged some cash but really no need for cash.
Very cold and windy. Some locals have advised if you rent a car, you may consider to take insurance for the paint of the car as well because the wind at some beaches so strong, the sand can damage the paint of your car.
For aurora, I believe you need wide angle lens (20/14mm...) with large aperture. the aurora is magnificent event to see and it has embedded in my mind forever. If you have FF camera it will be great.
And warmest clothes you have
For the diamond beach, I have learnt from someone to bring rubber boots, water-proof trousers, sticky tape. Wear the water-proof trousers outside the rubber boots then tape the trousers hard to the boots. By this way you can go to to water to take photos of the ice on the beach. Otherwise you need to stay dry away from water.
Hope this help.
Cheers and have a great preparation,