5 should be good enough, more is better, but 5 will do. discus do not need to be kept in even numbers...they can't count so they won't know the difference. I kept anubias without any problems but that is the extent of my experience with plants at that temp. I think you will find both hans and kenny ship discus in such a way they would be fine going to the arctic.
-careful with the angels, I kept two with some discus and they are such quick and aggressive eaters compared to discus it becomes hard to make sure the discus are getting enough.
looking forward to the pics!!