One thing you might want to consider is the potential security issues you take on by installing software on your Web server.
When you install software such as forum or blogging software on your Web server, you are taking on the responsibility of checking for security updates to that software and applying them as needed. For some reason, Weblog and forum software seem especially vulnerable to such security issues. For example, an older version of WordPress allowed an attacker to take total control over a site that had it installed, uploading and downloading and modifying files at will. I have seen eBay and PayPal "phish" sites set up on servers with the vulnerable version of WordPress installed. The hole has been fixed, but some WordPress users have not installed it--and hackers have automatic tools that scan the entire Internet, looking at tens of thousands of servers an hour searching for that vulnerable version of WordPress.
I'm not saying you should not do this, of course; I am saying that any time you install software on a Web server, you should be aware of the potential security issues, and you should always keep on top of that software, and be aware of any security patches which are released.
Another route which you may want to explore is using an existing blogging site like LiveJournal or MySpace. With LiveJournal, you can "embed" the journal in your site--it is still hosted on the LiveJournal servers, but it looks like it is contained within your site. Whether or not this will meet your needs is a question only you can answer.
