Since you have existing sites your choices are somewhat limited. cPanel and ISPConfig (to name a few) are meant to be installed on 'empty' systems. I'm not sure about Direct and Plesk, etc.
Webmin is good and you can set it up on existing systems.Out of the box it's meant for the root user to manage the site, however. You'll have to manually create lesser priviledged users if you want your clients to be able to manage their own stuff through it.