related links in pages

Hi,

do you have any idea, how to easily add and manage links to pages with these conditions?

1) I have 2-level page structure, let say 6 pages, each with some subpages
2) I want to link
a) existing pages or subpages
b) some pages that are not listed in the menu structure
c) files to download
3) I want to display these links in sidebar
4) Each page/subpage can have different links
5) If there are no links defined for subpage, this subpage inherits links from parent page

Are there any plugins for this? Or is it possible to use some internal WordPress functionality?

Thank you for any advice

Begis