1850 is often considered the most competative when it comes to tournaments, and anything above 1250 is considered "large" by most. I can assure you at a 1500 point tournament most people can fit backbreaking formations that will really cause some pain, hell, most people can field them at 1000 points or less. If you want to be competative you NEED the new wulfen book especially if your running a tzeench themed army, the tzeench part of the book has possibly some of the best warlord traits in the game, their new artifacts like the 'impossible robes' are also legendary and an almost must have on heralds and especially
LoC, also you need the book because they changed most of the spells for tzeench, giving them some nice upgrades and 3 completely new spells. I highly reccomend you take a look so that your army is up to date. Unless its a casual fun tournament then just have fun and disreguard anything I say
Even if you choose not to take formations, which is personal preferance really, just make sure to check out the warlord traits, spells and artifacts for tzeench. Makes it one of the best momo-god army