How to implement searching?

we are trying to add searching to our web site and want this search function to search only a few things:

• Files - Obviously we need to parse the text of PDF, PPT and DOC files in our case
• User comments. Users will be available to comment on the stuff and we want to catch if some user is searching for that relative information

I would like to have your advices on how to develop such a system. Should we get an external search system such as Google's or just install an open source one or develop a new one from scratch.

Thanks.

Check out this one: http://lucene.apache.org/solr/