DNS services have to be correct or OS X Server tends to get wonky and weird. That doesn't usually involve large transfers, assuming your server isn't serving DNS publicly.
The only reason to use forwarding servers is to delay the DNS translations, unless you really think you'll be lucky enough to have translations right out of the ISP DNS cache and also not right out of your own local DNS cache.
As for usage, Software Update Server (SUS) will consume four gigabytes pretty easily, as can various application-level updates, as can a local update that went directly to Apple without using the local SUS update cache. Xcode and some apps and an OS X update and pretty soon you're looking at real bandwidth usage...
Remote network activities on open servers can also require network bandwidth, such as remote accesses to web servers, or to any exposed files or resources, or folks uploading or downloading files.
There's unfortunately no what-just-used-the-network-bandwidth log within OS X Server.