Taking it a step at a time:
Everything you have done so far is ok. Everything else is in the Network -> NAT/MAsquerading section.
You need to masquerade your eth0 network to the external interface of the firewall. To allow traffic out other than http through the proxy.
You then need to define the internal servers in Definitions -> Networks then go back to the Masquerading menu and define a DNAT from Any External to the external interface through to the internal server in the DMZ.
Then setup a packet filter rule to allow Any on the service port required through to the defined server in the DMZ.
You will have trouble accessing servers in the DMZ by domain name unless you can map the domain name to the internal IP address of the server either using hosts files or running an internal DNS server with forward records for the domain that you control.
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