BuddyPress.org

It doesn’t matter if you install WP or BP first.

The only thing you have to take into consideration before upgrading BP is to deactivate any other BP plugins you’re currently using:
https://codex.buddypress.org/getting-started/upgrading-from-10x/

If you’re upgrading from WPMU 2.9.2 to WP 3.0, read this guide by Pete Mall:
http://developersmind.com/2010/06/17/upgrading-wordpress-mu-2-9-2-to-wordpress-3-0/

It’s also a good idea to backup every step of the way. Before upgrading to WP 3.0, backup. Before upgrading to BP 1.2.4.1, backup.