Welcome!
For ultrabudget builds I think cardstock pepakura gives much better results than EVA foam builds.
For normal builds I prefer sintra, as being a thermoplastic it allows for some heat forming and it superglues and sands very easily (wear a respirator with a good multi-gas filter though, heating or sanding PVC is not lung friendly). I have seen some amazing cardboard builds too though, and other materials, it's really just a matter of what you're most comfortable working with.
For painting, rattlecans are perfectly fine for trooping level jobs, some of more accurate colors though can be glosses, so you have to clearcoat flat after to equalize it all out. If you already have a compressor and airbrush, then you can of course go with that. Acrylic and enamel can both give great results, but personally I like to spray enamels and brush acrylics. Have good ventilation and wear a mask.
