Nagash Army?

Hello, I am very new to AoS, learning the Lore and looking through the models, and am deciding on my first army. Currently i'm leaning towards Grand Alliance Death with Nagash leading the army! Now I need to choose which Death army to collect first. The Ossiarch Bonereapers are my favorite with Nighthaunt and Soulblight Gravelords following.

How would I build an army with Nagash for each Grand Alliance Death Faction?

Which Army does Nagash lead best?