Here I am, a little late, but this Question came up in my search, and I thought I could drop two links about it.
A plugin that could prove useful is SearchWP. But, if you would like to read a bit and write your own code, you could take a look at James Collings' blog, which contains a lot of useful information and hints.
Note: When reading Collings' article and copy/pasting his code into your own, don't forget to change the names of WP tables to a more portable format; for example:
" . $wpdb->prefix . "postmeta
And, of course, put a
line at the beginning of the functions using the prefixes (otherwise PHP won't know where to go to load that