1. LK is Lord Knight, people just type LK because it's shorter, and No, LK's aren't the best class in PvP. Every class can be the best class for PvP, It depends in your equips and your ygg spamming skills.
2. For me, Champions are the right choice if you want to MvP constantly, because they can do a huge amount of damage which is called Asura Strike. You just need to practice in order to spam it.
3. People use Yggdrasil Berries or Yggdrasil Seeds to recover their lost hp and sp. They probably got Kiels in which reducts the casting time delay after you cast a skill by 30%. You just need to practice in order to spam faster.
4. In order to become an elite, your base and job level should be level 255, and Yes, Save your money for the elite set because it's the best set yet if you want to PvP, well only for Non-Donation I'd say.
5. Yes, people use food when they're high levels. It's called Yggdrasil Berry and it recovers your whole hp and sp bar, otherwise, there's this one called Yggdrasil Seed which recovers half of your hp and sp bar.
6. Coupons are. I think people only use zenny for broadcasting, well that's for me, but if you want to buy MvP cards or some good stuffs for your class, you need coupons for it, because people usually seeks coupons for it.
7. Dragon Aurora is a equip which occupies the middle slot in Equipment window. It's like a yellowish-orange dragon thing that orbits you while you are walking (something like that) and also adds +5 Stats. (If I'm not mistaken.)
I hope I answered some of your problems. :3