Just bear in mind that, unfortunately, i wopuld 80% of weathring or damage effect cannot be done by using airbrush. Mostly, brush and paint, pastel, cutter; other type of extra material like salt, plaster, sand, etc.

so u do not need to wait for tips on weathering before you ab (except one type: which is u need to cut holes or make uneven surface before ab). Most of the effects are done after you ab. So go ahead with your ab first, then only study/research more on weathering.

Dont let weathering stop you from starting to ab. mebbe u can show the after ab result, then some other would have better ideas / suggestion on weathering.

the base paint is either enamel or lacquer, it should have any problem. I suspect, either the base paint is not dry enough, or he spray too much top coat, bubble formed

It's very subjective, because details are just for decorative mainly, and not for functional purpose.

So base on your own judgement and personal preference. Normally, from PE (thinnest), 0.3mm, 0.5mm, 0.8mm are my own favorite. used 1mm too but quite rare. Never tried anything thicker than that.