try these sites:
http://www.hotscripts.com
http://www.cgi-resources.com
http://php.resourceindex.com
I know I saw a blog hosting script on one of the above.
I have met people online that have owned blog sites and they both are not in business any longer because they could not make money off of them. They both had over 20,000 users too. I wouldn't do it unless I had some investors lined up. LiveJournal.com just got bought by SixApart(i.e. Movable Type) and I'm sure they got rich off that, but they've been around for several years. You won't make your money back overnight.