How would I set up a model to be one of those fps weapons?

Make it a screen entity. From the manual (search for type):

ENTITY* shotgun_onscreen =
{

x = 100; // 100 quants in front of camera
y = -20; // to the right
z = -20; // below

type = "shotgun.mdl";
flags = SHOW;// visible on screen from the start
view = camera; // same camera parameters as the default view
}

It does not react on level lightning, though.

Is there any way to make it a little more dynamic? It is sitting on the screen, but I want it to rotate a little slowly compared to the player to make it seen like the player is actually holding something

... You did not do too much in 14 minutes to try it yourself but here we go:

function smooth_weapon_startup() {
while(1) {
shotgun_onscreen.pan += (mouse_force.x-shotgun_onscreen.pan/10)*time_step;
shotgun_onscreen.pan = clamp(shotgun_onscreen.pan,-5,5);
wait(1);
}
}

Not tested.

Well thanks, it works