Well, as my fantastic Controls professor used to say, "friction always acts to stabilize the system, so there's no need to model it." Particularly given the magnitude of the forces involved in a car suspension, you can leave those terms out.
Well, with the Morgan that stabilizing is actually significant and of interest - some people report managing surprisingly well without any shocks/dampers at all just because there is so much friction
My bigger question is -- exactly what are you trying to accomplish?
Basically just understand the system, and how different tweaks affect it. A good example is that a lot of people seem to shorten the springs in order to get a softer suspension - and of course accomplish exactly the opposite, except when the shortening drives the suspension in a mode where the rebound spring is fully extended and floating free.