"Andrei" wrote in message
in the house. Therefore, I got an idea to remove a few old windows,
enlarge the openings and install new, bigger windows.

Questions a Is it possible to do it at all and if yes, how
expensive could it be?

Can be done. All it takes is time and money.

Making a window larger by going longer (down) is easier and cheaper than
going wider. To go wide, you have to add a larger lintel over top to carry
the load. Not something easily done by a new DIY kind of guy. Brick must
be cut, some must be put back after the lintel, etc.
Ed