As for the weathering, I look for places where oil or other fluids might pool and leak, and then get blown away in the slipstream (I think that's the word I'm looking for). Exhaust staining and gun stains are a must, as is paint chipping. I nearly always make my models look dirty via earth-tone pastels, Tamiya weathering pastels, etc - I go after the tires and wheel wells and I will often go for the look of dried mud swept across the underside of an aircraft.
On the top side, I have often tried to vary painting among panels to depict sun fading or perhaps in-the-field repair jobs (pirating panels from non-functional aircraft to repair damaged parts on functional birds, for instance).
With my Stuka, I purposefully toned down the dirty look, but it's still gonna be a dirty warbird.