Multi-home: fix IP sub’ and DHCP via DSL

I am having problems setting up a multi-homed NT4 server to address a sub-net with fixed IPs on one side and access to an ISP with DHCP (NAT) addressing via xDSL on the other. It is intended to to set up a firewall and cache with Proxy Server 2.0. prior to exchanging NAT for fixed IP. Apart from the stringent installation requirements of Proxy Server 2 itself, can anyone help identify the connections necessary for my brain to get this arrangement up-and-running?