back to your question, now that i have more time to write
First, you need good and accurate maps, cause there are literally thousands of forest roads,so it`s very easy to get lost, some roads has signs some not, better yet if you have GPS, i don`t use but i have 10 years of riding experience here,but good map is also essential for me.
Second, if you come with DR,acerbis tank at least and even better with 5-10 liter jerry can, especially in east Lappland gas is only in bigger villages.
Security, cell phone is essential, but in some places of Lappland cell phone don`t work and remember in some remote places it can take weeks before you see another man if you are injured and phone is not working
Beware brainless reindeers, sometimes they run in front of your bike like horny rabbits and sometimes they stand in the middle of road like Moses in the mountain.
Moose, it`s known a grown up man to crap his pants, when 2 meters high and 500kg of meat with horns runs from front of your bike
Bears and wolves are usually too scared to show up,so don`t worry.
Avoid western Finland,it`s too urban for offroad, Lappland and Kainuu are probaply best regions for offroad, my favorite places are in southern Lappland between Taivalkoski-Posio-Kemijarvi places.
Last remember not all Finns are monotone penguins like one anonymous formula1 driver Kimi R.
but maybe a member of the DR-Forum wants to come with me?
..listen to that funky music white boy .. 
The natives said that we would get used to it after 3 or 4 weeks, but my hiking tour lasted only nearly 3 weeks 