First of all, “Enterprise” and “Standard” are different version names given to some distributions; most distros do not make such a distinction, others may have several different variants named completely differently.
I wouldn't necessarily use any of SuSE, Redhat or Fedora to run an ISP.
Personally, I would use a dedicated firewall distribution on one box, and a server-oriented distro to run email servers, web gateways and whatever else you want. A major distro like Fedora might help you here, in that it will provide lots of useful servers in one place.
You will find a lot of linux distros at
http://www.linux.org/dist/
Hope that helps,
—Robert J. Lee