I agree with planetphp here, although I think there are certainly many reasons why you'd want to roll your own.
Personally, I build mine from scratch on a website by website basis, but that's because I know I can, I'm aware of the security issues, I want it to be bespoke and simple and I have too much free time on my hands! From the sounds of it you're requirements are nothing that doesn't already exist in Wordpress (or any other CMS you'd care to mention), so you'd be a lot better off I think going that route. It means that you'll often have a community of support behind you (depending on your choice of off-the-shelf CMS) that you can add to your own knowledge of coding, and you won't have to get bogged down with the mundane/fiddly issues, like general security, concurrent access, etc.