You still have 7 months of time to get to know whats going on in woofer world. At the moment most recommended woofer sizes seems to start from 15", which also should be enough for any normal Finnish home.
XLS12 is quite good woofer, but its recommended to port or passive tuned enclosures.
Here is my version of it. These sub measurements also are one to watch when you try to find what to build. Those have proven couple starting trends in woofer worlds to be valid.
Also you should consider asking same question in Finnish forum @
www.dvd-plaza.fi . You'll get little more local information where to by woofers and amps. That forum is also the another home of Zipman, guy who made those woofer measurements linked above.