Adding additional fields to comments isn’t hard. There are tons of decent tutorials out there (and probably prebuilt plugins too). Google it. Here’s an example: http://wp.smashingmagazine.com/2012/05/08/adding-custom-fields-in-wordpress-comment-form/
As far as I know, searching on tags + categories + comments isn’t possible. I’m confused on why you’d want to search on comments?