2 networks are necessary for your LAN.
internet - router ---- outer network --- firewall - router- proxy - inter network(your PCs).
the outerwork and interwork will use differrnet network address.
The outer one will use the IP address assigned by ISp, and you manage the your own IP assignment for your inter network.
you can use DHCP in your inter network but you don't have to use DHCP in out network, just static one.
Put your web server in outer network, config your router to open ports 80 to you web server. the same to your email server for security.
If you don't have enough IP for your all pc, you have to use proxy.