The first time I played him, I took 4 Undeath, 2 Vamps, 2 Light, and 1 TK. I found the light spells to be somewhat useless and I got the KB spell from TK, which I did not end up using. The second time, I took 5 Undeath, 1 Vamp, 1 TK, and 2 death. I think this was better, but I ended up getting the TK movement spell.
I think that it depends on which scenario you get and what kind of list you are running. I was also running a MN with BP and casket for both lists, so not all of the spells got attention.
Nine spells is already a lot to use in a magic phase, so I think having Lore of Death to refill your pool is important. If Nagash doesn't take Death, than having another caster that does will likely suffice, but may not be as effective.
Undeath is important because of how Nagash buffs it so much. Granted, it's not imperative to take Undeath (save Ryze), but it would be silly not to; wounding your enemy's army is important, but overwhelming them through sheer numbers makes that easier. The fact that Nagash can more than double an army's size, both in points and models, is a huge deal.
Light is more augment than anything, but it doesn't augment the attributes that really matter (S and T) when it comes to fighting. Attacking first or on a 3 isn't that great if you still wound on 5 or worse and don't ignore any armor.
I am not that familiar with the TK lore, but it seems like it would work well with VC units, primarily the extra attacks, re-roll to hits (shooting too), and KB spells.
The VC lore is much less effective for Undead Legions than it is with VC. Curse of Years is still great, but not the ideal spell for him to get. Wind of Death is ok, because if Nagash misfires on WoD, he gets no saves, so that's bad. Granted, it won't kill him if he's at full health, but it could do serious damage if he is at full hp or kill him outright if he's not. Raise Dead is useless for Nagash. the only two spells that I am looking to get with Nagash are Hellish Vigour and Vanhels. If I don't get one of those or deem it tactically unnecessary, I take IoN.
