You will not need to add any weight to the AR cement hoppers as they are nice runners as built. I do not know, or care, if they meet any weight standards set by various mobs.
I have quite a few of these unit covering several generations of 'company' logos and all look pretty good on the layout and they do not take all that long to put together as an out of the box kit. If you wish to detail them to a higher standard of finish, it will obviously take longer.
To get a good weathered finish, I simply airbrush them with several light coats of different greys. I use Tamiya paints....XF66, XF54, XF24, let them dry for at least a week or until you can not smell the paint and them dribble metho over the top and then down the sides of the wagon. The end result is different on each unit.
Put all your decals after the first couple of layers of paint and then one or two very light coats over the top of the decals.
The hopper body can be built seperate from the chassis and I glued all the hatches and the like from inside the body....
Because of the seperate chassis and body, you can fit a rear flashing light to the wagon with ease and all the circuit board and wiring tucks neatly inside the body.
Have fun with then and if you are not happy with the finish of any unit, wash it clean with metho and do it again.
Just take your time with the weathering and don't go too hard.
You can paint the unit with a metalic paint and then go with the greys but I did find going straight with the grey paint better for me.
Regards and respect