Sorry Mark, I wouldn't know where to start on this.
All of the big search engines developed their own search engine software or bought it from someone else (Google, Altavista, Fast etc.) I haven't seen any open source search engine software, but then I haven't been looking either.
A small but important point: you are going to need a mighty computer system to properly index even a fraction of the internet. At the time of writing Google searches 3,307,998,701 web pages. They say that 3 billion web pages translates to approximately 3 trillion words, which is a lot of hard drive space!