it all depends
It all depends upon what frequency range your signals are in and how accurately you want to model the switching speed (on-off) and throughput.
For lower frequencies a hand calculation based on data sheet parameters of R and C will work. For higher frequencies SPICE is adequate. For mundane (consumer products) microwave use, Genesys and Serenade are adequate. The higher priced simulators are only needed for higher frequency commercial and military product simulations.
As far as models go, each simulator comes with models. Agilent seems to not give out models in order to "force" people to buy their simulator. The market acts otherwise. People buy parts that are in their simulator library.
You can go to the Agilent web site at www dot semiconductor dot agilent dot com (use A for @ in the above) and click on the quick link area application notes. You will probably find a note on the circuit you want and just copy the example design in the application note.