Hmm interesting. I might have to check it out.
1. On normal difficulty, the campaign can be anywhere from eight to ten hours I would say. It might take longer for someone not versed in FPS games.
2. You gain credits, which are used for armor, from pretty much anything you do in Halo: Reach. The amounts vary, and a lot of credits can be gained in MP or by completing daily and weekly challenges. Challenges are in all gametypes. in short, playing MP will get you credits faster, but it isn't required. Keep in mind that the armor "upgrades" are strictly cosmetic and have no effect on in-game play.
3. Reach is actually a perfect place to start. It takes place just before the events of Halo: Combat Evolved. If you were so inclined, you could pick up with CE right after finishing Reach. Knowing canon from the other games helps a lot, but I don't think it's necessary to understand the storyline.