That is one of my areas of expertise. I have written quite a few CMSes (6 different ones over the years, i think), and used/adapted at least a dozen different ones for different project. I ended up writing a CMS so many times because most only look good in the surface and are either unfriendly to developers, designers, or end users... And I have literally tested hundreds from red dot (costing millions) to open source ones.
I found out that there are many good CMS-es on the front end but almost all fall down rather badly in the admin interface - it is confusing, complicated, heavy, often slow or ugly. And before people jump in saying the one they use is good, imagine trying to explain to your mother or grandmother how to use the admin to add articles, organise the navigation etc. (The other area of weakness is often templating, making it very slow and difficult to adapt or greatly modify a site). Incidentally the best interface i found so far for a simple CMS (outside internal products which i can't show) is website baker. I think it's so simple a client given a website on it would actually update his site often. It's not a tool for what you are trying to do.
What I did was try the online demos of the admin of dozens and dozens of systems to find one that was intuitive. Surprisingly a lot of popular ones actually have very quirky non intuitive interfaces - people learn them and put up with them...http://www.opensourcecms.com/
Has demos of a lot of those, and you can in a few minutes have a look and rule out quite a few.
Anyway on to a few which were mentioned
Expression engine is actually quite easy to admin, although I don't like their templating when you try to do something complex like cross referencing blogs etc. It has a slight modular nature so you could possibly write an extension for your software downloads etc. It is more a blog than a CMS, in that it is heavily biased towards the "chronological" side of things, alhtough you can give more structure to things. Expression engine does have publish-on-a-date so it might help you, but it doesnt have the files support, you'd have to kind of do that via links or something.
Plone - I have worked on some plone projects and I must say plone is a good choice if you are pretty happy to stick to the way it works and looks. Try to do modifications of plone - whether to make it look wildly different or to add complex functionality and you will get huge headaches. We sure did. You are talking literally hundreds and hundreds of files in multiple versions with very complex "override" rules etc. Just Zope might be enough for what you need and that makes it already a lot simpler, but it is still an extremely complex system to debug and figure out. Plone does a few things right though, multi-lingual content is one of them, the use of TAL is another (once our designers tasted TAL we have had to use it on all subsequent versions of our own CMSes, finding or writing TAL libraries in whatever language we used...)
I recently used ezpublish and it's a bit quirky but it does support the kind of things you would need for your site - the ability to have custom content types, the ability to schedule release of new versions of the document, uploading files within the cms (so they too can have a release date). It is quite a heavy system, and needs php4, so probably problematic on your host. But if they have a "fantastico" style panel they might have it on there. It's heavily CSS based so playing with layout is quite nice. I found the templating quite hard to get my head around (works on combining lots of snippets, with context based overrides etc., so chasing problems reminded me of plone) but the html is actually very clean.
spips and typo3 are two more i looked at recently for a project and spip especially seems quite extensible, but i didnt like the way the templates worked.
I suggest you pick one which is clean and simple, where you can see how you could build the content on your site on and clearly see how the database fits together etc. Then you can add the automation yourself behind it.