You'd have to build him right, I think. He can be taken in a 2K list, unless I'm missing something, which means you have a 500pt army backing him up (500pts for the core - and as pointed out, the core sucks, so can't really be counted for much IMHO).
This means you need to encourage as much magic as possible - hence, Casket of Souls and heirotitan are pretty much autoincludes.
You're going to want to be able to bring on all of your models available, which means that you're going to want to take every Undeath lore spell to guarantee you don't miss out on an important one - so take all 7, leaving you with 2 other spells to randomly roll - I'd take both for Lore of Vampires so you can heal yourself should you get injured, and for Invocation should that prove necessary, or Death so you should be able to get additional power dice (but honestly, doing damage is the last thing you're going to want to achieve).
Don't take any other characters, you can easily raise them (remember, he triples raised points, and adds 30pts for each counter spent, rather than 10 - So Ryse 9+ = 150pts Infantry, Ryse 14+ = 300pts Infantry, Ryse 16+ = 450pts Monstrous Infantry, Razkhar 10+ 225 War Beasts/Swarms, Razkhar 16+ = 450pts Monstrous Beasts, Kandorak 10+ = Character = 195pts, Kandorak 24+ = 600pts Monster, Chariot or War Machine, Akar'aran 16+ = 450pts of Cavalry, Monstrous Cavalry or Chariots). Although a BSB is fairly important, they're expensive - the cheapest one is 110pts for a Wight King - using Kandorak on a 10+ gets you a BSB you can place.
So, it's looking something like;
Nagash = 1K
Core = 500pts
Rare = Casket, Heirotitan = 310
This leaves you with a further 190pts to spend - This prevents you from taking Black Coaches, Terrogheists, Necrosphinxes, Mortis Engines, Blood Knights (200pts Minimum unit cost).
190pts for me would look like double Screaming Skull Catapult (180), and then max out archers for ranged damage, and then raise in tanky blocks of units - maybe a single unit of Zombies with which you can throw one dice Invocation of Nehek/Morkarn to add to at the end of each phase if you have it spare and your staff has 4 dice stored in it.
It would suffer to a faster cavalry based army until you can summon enough units to put down something to take it down (a Mournfang heavy Ogre army, or a Skullcrusher Heavy Chaos Knight), as they can easily force an Instability check on something as crap as Skeleton Archers with around give or take 15 wounds, causing entire units to blink out as a result of their horrific damage output.
Nagash doesn't come into his until 3K, IMHO, when he has a balanced 2K army list (remember that you're going to effectively have larger than a 2K list, as you're not going to want to "waste" points on your previously mandatory level 4, who can no longer cast spells thanks to Nagash solely making use of the winds of magic, so it's probably nearer the base army of a 2,4-2,5K list). These armies at least have access to thinks like Necrosphinx for Ogres, or Black Knights+Banner of the Barrows to make a mockery of heavy cavalry,
I think he's pretty powerful, but I think the problem will become using him as a crutch. For a beginner, I'd recommend just learning the game originally. It would be like suggesting to a new 40K player how to use a Primarch, who brings all these massive force multipliers to an army (literally, in this case). Sure, it makes them individually more powerful, but it makes the army massively smaller (in an army that relies on its size over individual valour for the most part), and removing such a crutch completely changes the status of the army.
I.e get him, paint him, it's a beautiful model, and even play a few games with him - but while you're learning I'd keep to a more "normal" army.
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