Greetings from Helsinki.
Do you want to brew all-grain, extract, or in a bag?
If you want do brew all-grain, then you'll struggle to get a full rolling boil unless you have a gas burner with an output of 8kw or greater; you can't really do a 20 litre boil with a 5kw burner (the size usually sold in garden shops for paella cooking) unless you use a lid and that will impart bad tastes to the beer because the sulfur steam can't escape.
Here is a suitable burner that can be ordered from Belgium:
Since large kettles are extremely expensive and hard to find in Europe, you'd be better off buying an old 30 or 50 litre beer keg and cutting the top off along with installing a spigot on it. A 30 litre keg will give you enough room to do a 20 litre batch without risking boilover.