I don't know a 100% if Finnair has upgrades for M class. Last year I tried to find out (flying HEL-SIN), but if I remember correctly an upgrade would have cost at least the difference between a business and economy class ticket.

If possible, call Finnair sales and reserve in advance seat 22H in economy or an emergency exit seat. They have lots of legroom. But of course it can't be compared to the business class experience in seat 1H or 1G.