You can run some types of servers on OS 9, but don't expect any kind of reliability or versatility. I would strongly recommend upgrading to OS X or Linux if you want to run a server that other people on the Internet will actually be accessing. Your brother's iMac is fast enough to use as a medium-traffic web server, but is it connected to a UPS and do you make backups at least bi-weekly? What kind of firewall rulesets would you want to apply? Do you know how to edit Apache's config file to adjust which DSOs it loads? How can you adjust how often your logs are rotated? Will you know how to screen the logs for various errors? How will you prevent spammers from hijacking the server and using it as an open relay? Likewise, how will you prevent script kiddies from hacking it and using it as an FTP server for warez and DivX porn, and when it happens, will you be able to take responsibility? Etc.
I would recommend going to the bookstore and getting one of those big, thick books on Unix system administration, to prepare you to answer these questions before you set up and administer your own Internet-connected server... unless you don't care about availability or security or the likely negative consequences and just want to tinker around for fun.
Alex
