Auto Tagging with Suggestions

Hi there,
I’m setting up a new wordpress site from an old site and I’ve imported posts in from Blogger for it.

What I’d like to do is to be able to tag/categorise all previous entries in an automatic kind of way (its a large enough site) but to be able to suggest some tags it might use.

The overall plan is to have different sections of the site that read in the tags for pages. So say my main nav would have Topic 1, Topic 2 and Topic 3 and when you click on them they’ll bring up the posts that have the tags topic 1, 2 or 3. So ideally when the auto tag is happening it would put Topic 1, 2 or 3 on it as well as whatever other tags.

Is this possible does anyone know? Or am I going about this the wrong way?

All suggestions most welcome and thanks very much in advance for your time

cheers