It's hard to said...it's depend on your skills.
A good way to start is the doc:
http://docs.nethserver.org/en/latest/
Take a look, and you will happen a lot, and the Nethserver gui is clear, and easy to navigate.
For setup the Firewall/gateway, it's depend of your network.
But already read the doc, and you will see if NethServer feet yours needs 