How to Manage Multiple subdomains, subdirectories

I have a rather complex site. It needs to use subdomains and subdirectories. But the problem with using WordPress as MU for subdomains is that they are virtual. I need to have other scripts using that subdomain at the same time.

For example:

cars.mysite.com/ (for the blog)
cars.mysite.com/directory/
cars.mysite.com/forum/
http://www.mysite.com/bluebook/
other.mysite.com/guide/

So you can see the issue. Right now I don’t know how to manage the “cars” directory since WP would need to be installed in the www and then virtually use the cars subdomain to create the blog on that cars.mysite.com subdirectory. But I also need to create a directory within cars to install a directory that will run there and also a forum. And then there may be blogs in other subdomains.

Unfortunately this would seem to necessitate multiple WordPress installs. I wonder whether there is an easier way to manage this or whether there is a plugin that can synchronize all the users since I don’t want to have users needing to register for each part of the site. Single Sign ON would be ideal but I’m guessing this wasn’t part of the original plan for WP. Anyone have ideas on structure?