Couple of important issues before I migrate into Drupal
Taxonomies and pathauto/token will solve your first two questions. Use a taxonomy to categorise your nodes, then use a token such as [term] or [termpath] to add it to the start of the automatically generated URL paths.

The sections module will solve your last question. You can assign different themes based on paths, or for individual nodes, and some other ways too. Unfortunately, it looks like it doesn't currently have a released D7 port. There may be one in the works, or you may need to try to port it (likely not hard from my understanding of how it works). Possibly related project is taxonomy_theme, which seems to have a -dev D7 release. I haven't used this module in either D6 or D7 personally though; I know sections works fabulously though in D6 despite not having an update in 2 years now.