Nagash is awesome to paint and use, he does limit some list options tho due to his high points cost.
U definitely need 3 battleline choices, in GHoN Morghasts are battleline so there's 1 as you've said u like them, (I love these things with halberds). Take a unit of dogs, they're cheap and perfect for nabbing objectives. That leaves 1 spot, take either chainrasps or skeletons in a block no smaller than 30 as you'll need at least 1 big block for objective holding, I would choose chainrasps personally, more durable and fly.
To support nagash a necromancer is a must for van hels plus amping nagashs spell pool. An additional hero (I'd pick vamp on steed, cheap but effective) is optional but again adds spells to nagash and can take character only objectives. For a super spell aggressive list u could take arkhan with nagash.
Whatever points are left should go towards an elite unit choice, the concensus is that grimghast reapers are the go to choice in most situations but u can just double down on heroes, morghasts or skeletons/chainrasps if u want.
Hope this helps, pick the stuff u want / are required to have and work around that. Welcome to team death
