Welcome to the Eye of Terror and the Legions of Chaos
a.World Eaters or Mostly khorne army: Easy tactically, as mostly you charge towards the enemy and hit them with a variety of pointy objects. Medium-difficult to paint, as getting the right shade of red can be hard.
b. Thousand Sons: Tactics are fairly simple as long as you're facing MEQs. Versus hordes you will be in more trouble. Painting can be difficult, as they have a fair amount of detail and I've heard people complain about the helmets
c. Iron warriors: Tactics are basically stand and shoot until the enemy ISN'T standing. Easy to paint, as a simple black undercoat with a silver/gunmetal drybrush gives great results.
d. Chaos undivided: Could be the easiest to play, as YOU dictate the way the army plays. The painting really depends on what YOU decide to do with them. Might be the easiest choice, as you could paint them any color and not take any flak for not following an established scheme.
2. Obliterator and maybe more rhinos. A defiler or two always improves an army, too.
3. Check out the rest of Heresy Online, especially the Modelling and Painting
boards, for helpful tips.
4. Any army can have Ahriman, but he works best with TSons. Having said that, I'd recommend NOT taking him in a force of under 1750 points, as he IS a points sink for what he can do.
5. Not at all- I routinely field a few dreadnoughts in my TSon armies. I especially like the one armed with twin CC weapons.