As with G5C, choose spells that provide abilities not found in black, and require only one colored mana. Disenchant and Incinerate are almost universal. Some decks add additional firepower in the form of X spells or Earthquakes. Arcane Denial is an obvious choice, but few multicolor Black decks use it, sticking Hydroblast and Pyroblast in the sideboard instead. Like Nekrataal, it merges a powerful ability and a decent creature into a single card.
Many decks stop at four colors.
This is mostly a metagame choice. In the environment at that time, Thawing Glaciers and blue-white decks with Kjeldoran Outpost were everywhere, so Choking Sands worked well. Most use twenty to twenty-two lands. However, they don't need to focus so much on basic land. Be careful not to include too many painlands, though, or you'll sting yourself to death.