Well for me personally, I dont have an exact recipe for lightening Tamiya's Dark Yellow. I just add white till it looks about right. IMO it should really look like a light tan when you are done.
As for airbrushes, I myself use an Iwata HP-CS for almost everything I do. Not the cheapest, but I started out with an old Aztek airbrush. I am by no means an expert on AB's so I would defer you to some of the members here that know much much more about them.
For treads and roadwheels, yes I paint them. I assume you have the rubber band tracks that came with the kit?? The rims of the roadwheels are rubber so a shade of black is what most use. Alot of folks like to use some darker grays for the rubber and is looks more realistic. Some folks do it by hand, but there are some of us that use a drafting circle template. Paint the entire roadwheel the tire color, then you can come back and place the circle template over the roadwheel, effectively masking the rubber, and paint the base color on the center rim.
I am sure others will be along shortly to give you thier advice also. There is alot of talent here to be learned from to be sure!