Hi Mark,
There are several plugins for wordpress that you can use to get around this dupe content issue with tags, categories, author pages etc. The fact that you are running a hosted WP and not your own install means you can't add these extra plugins as far as I am aware. Again using a hosted WP you also can't change the URL structure to a custom one.
So you mention you have a 17 year old site. Is the setting up of the blog on WP to drive traffic and some links to your main site?
Have you considered adding a wp install to your own site, and ading those 18 articles you've written to your site, that way you can take advantage of all the great SEO plugins and optimisie everything as needed.
Some SEo plugins for WP i like are
http://yoast.com/wordpress/seo/
http://urbangiraffe.com/plugins/headspace2/
Derek