Lotsa questions, but that's cool, as i'm in the same boat as you (have a couple armies, really love the Legions of Chaos idea)
My take is that 1k out of 2500 is a LOT on lords unless they provide a force multiplier. Luckily , both your lords do. They can both cast the summoning spell from Glotkin book, so therefore can both act as a force multiplier. If every turn they can summon up a hundred or couple hundred points, over the course of the game they might be able to completely reduce their base investment down to net zero (albeit with some luck), or at least reduce it significantly. You'll need to plan for this accordingly, of course, that you're reducing the initial investment damage from taking two big honkin' lords by summoning, and tailor your plan to that.
In terms of mix of units: if you want to use Tamurkhan stuff that is NOT listed on the legions of chaos, you'll need opponent's permission. The listing of allowed units is pretty specific. They might errata it, but i wouldn't hold my breath. If it were me, i'd come up with a
LoC legal list and a "side bar" of the tamurkan models i want to use if my opponent is cool with it. Around here people are typically cool with stuff like that in a fun game as long as they are familiar with, and can have immediate access to if requrired, the rules. Didn't bring your tamurkan book, and want to use plague toads ? Sorry, no dice. That's just me though.
Your thoughts on composition mirror my own : beastment for fodder,
WoC and
DoC for elites / cav. Only difference is i LOVE doombulls and minotaurs, so i've been playing around with methods of getting some of those into my lists.
As for exact composition, i haven't gotten as far as you, but i'm with you that i want to use a mish mash of things - i'm going to focus on nurgle so i know i want Plague Bearers, Plague Drones, Blight Kings, probably some Gors, ungor raiders, and then some things from
WoC (cannon for sure, and warriors). Fill to taste with cool stuff i like.
That's the best thing about End Times, you can do some really awesome and cool things with it. I really hope they do something with Ogres.