I would do this.
Make your Top level design and pages. On the "subSites" just apply another design(skin and/or containers). This means that you only have one portal and only one login. But to the users it will appear as different sites.
Reuse content. Try this:
1 In your txt/html choose export content.
2 On the new page insert your txt/html module and then import content and choose the xml file you exported in step 1
OBS if you have put in some text manually or have imported a worng text you cant import a new text. Then you have to delete the module and insert it again.
Extract from the manual about skins:
Host Level
Host level skins apply to all portals in your site. They can be assigned by logging in as the SuperUser and selecting the Host / Host Settings tab. You are able to assign a skin and/or container for both the public portal and private admin interfaces.
Portal Level
Portal level skins apply to a specific portal. They can be assigned by logging in as the Administrator for a portal and selecting the Admin / Site Settings tab. You are able to assign a skin and/or container for both the public portal and private admin interfaces.
Tab Level
Tab level skins apply to a specific tab in a portal. They can be assigned by logging in as the Administrator for a portal and selecting the Edit Tab Settings option from the tab admin control. You are able to assign a skin and/or container for a tab.
Pane Level
Pane level skins are actually module containers which apply to a specific pane on a portal tab. They must be configured by the skin designer when creating the skin and cannot be managed through the portal user interface. Module level skins assigned to a specific module will override the Pane level skins.
Module Level
Module level skins are referred to as containers and apply to a specific content module instance on a tab. The can be assigned by logging in as the Administrator for the portal and selecting the Edit Module Settings option on the module Actions menu.