You should also consider Skaven as an option - they have some of the best artillery units in the game and lots of fun monsters.
Their magic is also really strong
Endless spells are not part of any faction/allegiance, you can buy them for any army
So in a Grand Host of Nagash Army, just pay the points for the ghoul spells and Nagash can cast them (or an allied ghoul king), but necromancers and stuff can't.
You dont actually have to reroll first and then modify anymore with crypt horrors.
With the new warscroll they can reroll all their hit rolls as well as their failed hit rolls, so you can just reroll anything that isn't a 5+ vs dryads for example.