Heikki,
Old Singers are wonderful machines.
For doing some basic sewing they are hard to beat since they are made out of metal and are made to last. I have a few myself.
The two main machines I sew on are a Singer 401a and a 101.
The one pictured on top is a real beauty. I just love the decals. I wish someone had given them to me for free.
They absolutely have value, but as to a dollar amount I couldn't say. There are many people if they heard someone had gotten either one of the machines for free would be looking like

and wishing they had such luck.
Looks like from what I generally saw the 306K should sew about anything.
To help you find out more about them, date & place they were made etc. the Singer web site is great. This can be found by checking serial number. I would be curious to know about your oldest one.
http://www.singerco.com/support/serial_numbers.html
manuals are still available, not sure what is available in Europe.
http://www.singerco.com/support/model_number.html
There are some articles on this blog that might be useful to you on sewing machine repair/inspecting older machines
http://blog.sew-classic.com/categories/Sewing%20Machine%20Repair.aspx
interesting site
http://www.geocities.com/possumjim/xhtml/collect_sewing_sing306.html
Yahoo also had a vintage singers group that might be able to answer questions too.
Also I don't know how many people sew in Finland, but I have a sewing machine repair man that works on older machines, gives them a good cleaning/tune up etc. and can explain how things work etc. Maybe there is such a person in your part of the world.