Specialised search form

I am developing a small intranet that will be used to publish briefs (news) to staff. The taxonomy has 3 terms in order of importance - Urgent, Essential and Other. The frontpage has been modified to display a list of briefs showing importance, node id, title and date. What I would like is a seperate search form underneath the breadcrumb trail that will allow the user to choose between searching based on
1) node id - node id is entered and the relevant brief will be found;
2) text between dates;
3) all briefs from the last 7, 14 or 28 days.

I know that the layout for this will need to be placed in the page.tpl.php file and I could probably write the SQL to perform the search, but don't want to change the core files. Would I need to create a function in the template.php file? If so how would I call it and display the output?

I am relatively new to drupal but have been very impressed with the structure and ethos of the system.

Thanks
TJ