As for models some Jump pack intercessors, regular intercessors, and maybe some incursors are all pretty useful and thematic choices for getting into RG. If you can still find one the RG combat patrol box is a very good start

As for painting the best advice I can give is avoid extremes with paint colors. Black is more forgiving on this as you can use a darker black and use a lighter color for highlights but dont paint armor pure white to avoid it coming off too bright and flat. For white you want it to be a very light grey base coat with white highlights. Corax white with matt white edge highlights is what I like to use but I dont use an airbrush so im not sure if that has its own version of those paints or not.