If you still think opting for a car, and as you are in no hurry getting to Suomussalmi, I would choose my route through the lake district.
Main roads 4 (E75) and 5 (E63) are extremely boring, and don't offer much. Skip those much as you can. Instead collect many of those smaller back roads that follow closer to lake shores, and through the countryside. Undulating, very nice to drive gravel roads in typical Finnish scenery. That's where rally drivers are born... :) And that's where our summer cottages are hid.
You were coming in July? Gravel roads should be in good condition then. Just get a good road map (GT Tiekartta for example). It's pretty easy to navigate through smaller roads as all roads are numbered, and numbers are marked on the maps and road sides - often at junctions. On a good road map you should also find some cheap lodging options.
For spotting some "meaningful national landscapes" on the way, check this page:
http://www.outdoors.fi/destinations/historyandculture/nationallandscapes/Pages/Default.aspx