Can't really help you with the ISA side of things, check MS's web site for config details. The config on the 3000 will be just a standard LAN-to-LAN tunnel, just make sure your Phase 1 and 2 lifetimes match whatever the ISA server defaults to.
If you only want to have access to one IP address, then yes, just putting that one IP address in the Network List on the 3000 should be enough. Again, just make sure you mirror the Local and Remote Networks (even if it's just one address) onto the ISA server.