I've been out in that part of the world a few times (-55 C was the lowest one we saw). I've used two setups, one was two -10--15C bags inside each other (one of each), and one was a Carinthia Surivival One (-40 C rated). In both cases with a polycotton bivy, a 14 mm closed cell foam pad and a reindeer hide underneat (GoreTex is strongly contraindicated in those temperatures; the vapor pressure differential is miniscule under those conditions, and it turns into a plastic bag). Both setups worked fine (well, at -55C the night was on the chilly side in the Carinthia), but the two bags system was messier to deal with, getting both hoods lines up, zippers, etc (today I'd probably sew some velcro or cotton tie ribbons on them -- say two attachement points "over your ears" on the hoods -- and join them up semi-permanently). If you can't see needing an arctic bag anytime soon again I'd say go the two bag route, else get the one lovely super-bag.
One more bit of sleeping kit that might come in handy; a smallish dry bag of some sort (not a heavy duty Baja bag, get a day-pack sized liner type bag). Use it to keep boots and such inside your sleeping bag at night. Then they stay un-froozen, but your bag stays dry. Putting on stiff, froozen boots in the morning is no joke. And pack your kit as for a summer trip when water crossings may occur; it is possible to go through the ice when crossing a stream... Just line your pack with one of those big black trash can liners (it will be horribly noisy and crinkly, but may save your bacon).
Now, one word of advice on sleeping bags. A synthetic fill bag is nice in one way (handles getting wet better), but it also has limited lifespan. I would not today, 15 years later, take my Carinthia out in -40 C if I had a choice. It simply has lost too much insulation value over the years to be comfortable in less than -30 C. I have a down bag that is over 30 years old; it has lost some down lately, but is otherwise pretty much the same as it was when I bought it (I need to open up a seam and refill it). Today I would probably buy a down bag if my money went that far (down bags for arctic conditions are expensive).
And for the rest of your gear; put on your big mittens and try to operate all parts of your gear; big loops on all zippers (say 20 cm of cord) help, check if you can adjust your backpack, open it, put in on and take it off, get things out of it, etc. Even tying your scarf my be a bit of a challenge at first. It is no fun not being able to get your warm hat on properly because you don't want to expose your bare hands to the elements.
Ohh, and Sundsvall is a coastal town, so it gets a more "raw" cold than the inland, unless there is no open water left in the parts of the Baltic nearby. I live about 80 km inland, and it is decidely different from the coast.
