1) always sand and prime a surface if you intend for the paint to stick to it permanently. If you're after a 'weathered' look straight off the bat, don't bother sanding, but the paint will chip off and expose the red below.
2) Halfords camo paints are excellent, their khaki and green are probably what you're after.
3) yes, but expensive
4) quickly, real soldiers don't spend hours making their guns look like works of art, they're tools.