WP will work just fine for your article-ish content, but when it comes to the store and the forum I would recommend just installing forum and store software then link to them. There are lots of WP plugins available for when you absolutely need something seamlessly integrated into your blog, but it's a much bigger hassle on the technical side. Since your website itself is actually a piece of software, you want your components to be loosely coupled as a good practice. Basically, you don't want one thing to ever be able to break another thing, so you don't want them "touching" each other unless you really need to. If you've ever written a WP plugin you'll know how ass backwards, touchy and severely limiting the process is.
Aside from that, the WP plugins available are almost never as good as what you can get standalone.
You can just have WP on your root, then have the directories for your forum, store and whatever else not be handled by WP. It's not hard at all. You'll have to look up the documentation for whatever server you're on but it will be a few lines of code and save you massive headaches in the long run. Just use directories instead of subdomains to keep your SEO solid.
You'll also just need to have a theme for each piece of software you're running, but honestly that can be a lot easier than trying to get your WP theme to style your plugins how you want.
