Port 80 is unencrypted, so I think it does not make much sense to do that. What you can do though is to change it to port 443 which is https:// by editing the ispconfig.vhost file, beside editing the port, you'll have to add a server name in that file.
I have been reading about it. Wouldn't it conflict with users' websites? Sorry for the doubts, I usually always worked manually configuring site by site without a panel.
No, because if you configure it as @tbrehm suggested, traffic for panel.example.com will go to your panel, and example.com will go to the webroot of example.com
