How do you get 3+ ward save Knights?
Knights make the better hammer
IMO. It's not just the damage output, it's the fact that they can arrive where they are needed more easily and more speedy and that they will often strike first. Of course it does depend on the target; against something that ignores armour Marauders are obviously better. Knights into Ghouls, Marauders into Grave Guard as an example.
A couple of Nurgle sorcerors in a mainly Tzeentch-themed army sounds like a contradiction, even if every unit has
MoT. What's more, multiple characters add up in point cost really fast, even if some of them are a small one-man army. Other than that, whether it "works" I think has little to do with the unit they go into unless I'm missing some nasty combo.