Methods for Dynamic Custom Content based on URL

I need some guidance on the best way to provide dynamic content based on the URL a user visits. Basically, we want a basic template page then for some images and text to be populated based on the url they visit. For example:

If the user visits
http://www.mywordpress.com/base/123 they should see images/text associated with id 123

But if the user visits
http://www.mywordpress.com/base/345 and this extension 345 does not have supplied content, the site should resort to a default url.

Are there any plugins or minor modifications we can make on the existing platform to allow for easy management of these pages? There will be entirely too many to maintain one by one, and content changes frequently.