Note that these general principles do not apply to blue or purple items. If you are a first-time player, there is absolutely no reason to purchase these items. None. Sub-80 rare and epic items are only for the alts or twinks of established players that have money to waste (because that's what it is) by showing off their Staff of Jordan (or whatever). Blues and purples are completely cost-ineffective for first-time characters. Within a few levels, you will find green gear that is roughly comparable, or you'll get better blues from instance runs at the same level. So, do not buy these items, under any circumstances, if you are a first-time character (no matter how cool they look). And even if you're reasonably well-off financially, think twice. This is especially true as your character gets closer to levels 58 and 68. Even the most basic quest-reward gear in Outland will have substantially better stats than anything you can buy off the AH for a level 56 character. The same is true of Northrend gear at level 68 vis-a-vis the stuff you'll get in Outland at level 68. As such, smart players stop making AH gear purchases by about level 54 or so, and then just gut it out until level 58. The same is true at levels 64-68. Within the first several quests in Hellfire and/or Northrend you'll have replaced half of your gear in any case, guaranteed.
Allied races are variants of existing races. The Void Elves are a version of Night Elves, while the Highmount Tauren are, you guessed it, a version of the Tauren. Four allied races are already in the game before launch and two more, the Dark Iron Dwarves and Mag’har Orcs, were added after Battle for Azeroth launched. Each Allied Race has its own racial abilities. You can also unlock a heritage armor set by leveling any Allied Race to the level cap.