How much modding experience do you have, luchtzak? I see your coppermine gallery is integrated. Did you do that yourself?
phpbb-seo isn't that hard to install. I think if you were able to do the coppermine gallery you'd probably be able to do it yourself, saving $ and learning more about phpBB in the process.
Work from backups, and we'd be glad to help you via the forum.
Sometimes it's useful to create a subdomain (for example dev.snookerforum.be) and work on that until it's working correctly, then transfer those files to the main domain. Do you have access to phpMyAdmin?