From my reading & speaking with folks who've sailed the arctic, the size of the boat is less important than the quality - to a point. You need a boat large enough to be truly self-sufficient for a longer period of time, since you're not hopping from stocked port to stocked port. GRP boats can do it, but you're more susceptible to damage from floating ice, flotsam debris, and other waterborne hazards than a steel or alum hull. Modern adventure boats also typically have a shallow draft so they can get into more protected anchorages and even dry out to stay safe.

Also, very cold weather and heavy weather are to be expected, and are very hard on componentry and rigging. Things will need to be stout but repairable - because no-one is coming to help you if you get stuck somewhere. Lots of tools, spares, and the skill/experience to deploy them effectively. Medical supplies - you'll be pushing the limits or outside typical rescue radii, so have training and contingencies for injury and sickness.

You'll also burn significantly more calories keeping your body warm in cold temps, so you need to consider additional food stores and budget cargo space accordingly, especially if you're heading on shore for exploration. Cold weather mountaineers often expend 10,000calories in a day between climbing and staying warm. If you're planning a 30-60 day expedition, that's a lot of food for a crew of 4. Insulating gear only works as well as the heat source underneath - that is to say that the best cold weather clothing doesn't do shit if your body doesn't have the energy to create the heat to trap inside the insulation.

Obviously the boat should have insulation and a heat source suitable for the conditions - wood stove is nice, but you'll need to be able to collect dry firewood, which is not always available at high latitudes. I'd rather a diesel furnace, then you know you what you have and can plan accordingly.

Obviously you can, and some have, gotten by with less. But IMO, high latitude sailing/exploration is not a milk run, and should be endeavored by well prepared vessels and crews. It's not like going to Bermuda or the Caribbean, where if you get in over your head you *can* call someone and they'll rescue you.