i'd keep the weathering subtle, especially things like rust. I paint the chassis flat black and use different acrylic browns to make it looked used. Use an old brush and stipple (dab) the brush on to make it spotty. i usually mix silver and different browns to depict different types of metal, like the exhaust system on my Ace Ventura Monte Carlo. adding a tiny bit of ultra fine dirt or pastel chalks to add texture.
Also, making the body paint matt, or mixed matt and satin gloss. the upper sun exposed parts usually fade first. Look online at pictures of junker cars for ideas. Plastic usually fades to a grey color. If you've got an airbrush, that is the best tool for applying faded paint and dust.
