Some personal experience.
Yes it is possible. I am building a site with roughly the same structure as you mentioned, and I am almost done.
In hindsight though, if I would do it again, I would perhaps not have chosen WordPress as my engine.
You would probably use different categories for the newspost of the different subsections.
You probably also want to mix static content with the dynamic content (?).
Which means you would want (or rather .. need) custom templates.
You already said you want different styling css/ per section, which should be doable with conditionals in your css indeed.
Perhaps you also want different users/usergroups who have restricted access to only their section of the site.
Not sure if you want to have the ability to make (selected) posts from the subsections also show up on the home section .. but that complicates things a bit further.
Still ... all of this is possible .. but you may have to invest more time than you anticipated to get it all working.
I ran into quiet a few issues, which for me all could be solved with custom templating and extra plugins.
The problem I have with using plugins for vital site functionality is that you have to pray the plugin wont break with the next WordPress release in case and/or that the author will still support it by then. If not, you will have to try to hack and update the plugin, or hope there will be another compatible plugin by that time that does about the same. For that reason I tried to rely as much as possible on plugins written by core wordpress developers.
Admittedly, you don't have to upgrade with each new release .. but you *do* want to upgrade when it concerns security fixes. And it would be a shame to miss out on new functionality. For example, I am looking forward to having the 'internal linking functionality' due for wp3.1.
Anyway .. this is *not* an Atahualpa issue, but more of a general WordPress issue. WP is moving more and more from a blog engine to a CMS, but its not build to manage multiple information streams and subsections gracefully (yet).
Here is an interesting related post: http://wpcandy.com/thinks/about-the-wp-vs-ee-pdf
. Note that the pdf comparing WP with EE is biased, it was meant as a sales pitch and was not meant as an objective comparison, but I think there is still some truth in it.
On the other hands .. wordpress has a huge user/developer base, a lot of momentum and I dont see it disappear in the foreseeable future. Some other engines are much more niche products, and you may well run into other problems with them, like lack of updates, lack of enthusiast users helping each other, lack of fancy plugins etc.
I hardly have any experience with other CMS'es like expression engine, concrete5, cmodx, habari or symphony etc.
But I think its worth to have a look other engines before making a decision.
If you go with WordPress, Atahualpa is arguably the best theme when it comes to flexibility and customizing.
Hope that helps.
In response to Paulae'sugestion to use WPMU.
I havent used WPMU .. as I thought it would complicate the integration of postcontent between subsections of my site.
But perhaps should have a 2nd look at it.
Anyway, these links might prove useful: