how to write a separate search plug in?

Hi,

I need to write a search plugin that’s completely separate from the existing search functionality of WordPress (I think).
Basically I would like to use the existing search function as it is, but have a second search function to search for matches after users have filled out a brief questionaire (this is a product based blog, so the users will enter specific features of the product they would like and the search will then only show them results matching their selections). I’m going to store these features in custom fields.

So far I have found a few plugins, that search custom fields, but the problem is that they EXTEND the existing search function.

What I’m wondering now is how to write a separate search plug in (should I reuse some of the existing search functionality, etc) and more importantly how to INTEGRATE such a plugin, as I don’t think there will be an existing hook I can call the plugin through?

Thank you for your input.

Steve