As far as the wheels goes I used two wheel collars per each tire. One mounted on the inside then the wheel and then the other collar (leave a space between here enough so the wheel can move freely.
The plastic bits I am going to assume is the Control horns. I used whatever came with the kit on this. If there are two holes use two if there is four use four. I did find that the kit provided most hardware needed but either I was missing a few screws or dropped them among the mess in the room I was building in.
Also, when you mount your landing gear the instructions call for you to angle in some screws and let the head of the screw apply pressure to the wire gear. I dont like doing this, so I replaced it with a plastic wire gear mount set ( I dont really know what you call it).