I'm not really that familiar with canoeing, but I decided to give some (hopefully helpful) comments anyway:
Travel:
Maybe you've already found it, but
here's our national rail company's route maps for areas outside Helsinki region.The company offering cheap bus tickets is Onnibus.
Here's their route map. (I think the SKI ones are only active during skiing season.) These days their tickets can also be bought through the Matkahuolto website.
I've only spent two hours in an Onnibus and didn't find it too comfortable. Wouldn't want to sit in one from Helsinki to Oulu (or even Jyväskylä). Maybe you're both smaller than me and will find it comfortable enough though.
Hossa:
Difficult to get to - the buses go there during school days, i.e. August to May, monday to friday (This year the first bus goes August 9). In June or July one would need a taxi from Suomussalmi or Kuusamo and that will cost 100 - 200 euros (we just got new taxi legislation starting this July (i.e. yesterday), so no-one knows how the prices will be next year.)
I managed to dig up some Kainuu area bus timetables, and the closest to Hossa one could get by bus right now is Peranka (only mon-fri, once a day), a radio mast and a house in a crossroads 30 km from Hossa. (Turns out there's a
Hossa taxi just for getting people from the bus stop to Hossa. No idea on the price though.)
Paddling to Hossa from Peranka (or Kuusamo, maybe from Suomussalmi. Google Maps doesn't really show how navigable rivers are.) is an option, but a plastic canoe is suggested for that. (
According to Suomussalmi travel guide. An inflatable kayak doesn't sound like it likes sharp rocks.)
Lapland:
You'll want June / (first half of) July, if you want to experience midnight sun. Though the summer nights wont get that dark anywhere in Finland before August. (I suggest you bring sleep masks.)
(
The Finnish Meteorological Institute's weather forecasts have sunset and sunrise times.
Here's the current one for Kemijärvi (some 40 km north from the arctic circle), showing that the sun rose June 4th and will set July 9th.)
Like Erkka mentioned, Lapland's known for mosquitoes and other blood sucking insects. Early summer the insect populations will be smaller so travelling will be more comfortable. (Though people live there throughout the year, so managing two weeks there shouldn't be impossible
)
If I was going to travel in Lapland, I'd want to do it in the first half of June.
Other location options:
My initial thought when I saw your post was to suggest traveling to Lappeenranta, renting a kayak, and paddling around Saimaa.
Another place to consider is traveling by train to Lieksa or Nurmes for
Lake Pielinen and the
Koli National Park. Pielinen drains to Saimaa through the Pielisjoki river, but I'm not sure how navigable that is. (There's the
Karelia Rowing Tour for church boats and small boats every July rowing/paddling in the area so it should be navigable. If you decide to paddle around Pielinen, and want to travel by yourselves you may want to avoid that event.)
In Lapland there seems to be good canoeing areas in
Inarijärvi (You can get to Inari by bus from Rovaniemi couple times a day) and
Kemijärvi (where you can get by train).
(By the way, I'll be moving to Mikkeli in the end of this month, so I might be canoeing in Saimaa next summer.)
Edit:
The National Land Survey of Finland has a nice map with topography/aerial images/whatnot.