Actually I have not got a solution yet.
What I need is a solution/template where I can define dropdown menus that viewers can choose from, and that would bring them to the next level.
In this case it is access to about genealogical databases containing scanned pictures of original genealogical archives (birth-, death-, marriage-, census-, probate-, and other original certificates and books.
The archive files are organized in a number of levels as e.g. County, Region, Parish, leading to a list sorted by date/year from which you can choose and open the individual book you can browse through the scanned images in.
Your suggestion to use WP, Squarespace or Joomla! might be the way to move, but I have no experience with either of them - perhaps you can guide me at little bit here