Hello cornergran
I went on a northern lights cruise a few years ago in February and really enjoyed it. Clothing wise I had some vests, long johns, lined walking trousers, a good quality coat, hat, gloves, buffs and warm socks. I also got some grippers to walk on ice which were useful a few times. Also I had fur lined boots ( a bit like shorter wellingtons) and some merrells. Some people just had regular clothes and seemed to manage.
I bought a lonely planet Norway which was really valuable. The excursions are pricey so the book allowed me to research the places we were calling at and plan excursions. So where there things to do where we landed I didn't bother with excursions to save a bit of money.
Hope this has been useful and if you want to know anything else just ask.