Mysql vs MSsql .. what ever is on the server. Mysql is easy, fast, and free. MSsql has stored procedures. Choose the one you like best.
As for writing a Google style new system, the biggest challenge is writing a crawller that can go off to websites and grab the right bits of text. The problem lies in writing it so that its flexible enough to be able to cope with the changes in layout sites throw at you. One day you'll be grabbing the text between divs with a #headline id, then the site will change their structure and your code has to be changed quickly and easily to cope with whatever they decide to put in. Some sites (like the BBC) make it a bit easier for you by having crawller friendly versions, but thats rare.
Flexibility is the key. And a willingness to spend all day writing new regexp code when Wired move their headlines two table cells to the left.
