Glad you find the CMS application useful.
One of the things we kept in mind when writing the book was extensibility. So: you bet you can add additional features like a search engine ;)
There are a few ways to add a search engine to your site. One is to use SQL Server's Full Text Search. Googling for this term brings up a number of useful tutorials.
Alternatively, you can execute a custom SQL statement against your database and then display the results. For a general overview of how this works, take a look here: http://Imar.Spaanjaars.Com/QuickDocId.aspx?quickdoc=241
I wrote that article as an extension to another book I wrote: Beginning Dreamweaver MX 2004 which deals with "classic" ASP. However, you'll find that the concepts (e.g. search term parsing, accessing the database, displaying results) is similar.
Chapter 8, the customer support site, uses the same strategy to search the site for downloads, FAQs and products.
Regarding attachments: take a look at chapter 8, 9 or 11. In these chapters, uploading of files is discussed. Once a file is uploaded, you can treat it like normal files; e.g. link to them, download them etc.
Hope this helps, and if not, please let me know.
Everyone is unique, except for me.
Author of ASP.NET 2.0 Instant Results
and Beginning Dreamweaver MX / MX 2004
Want to be my colleague? Then check out this post