I think you should have a general concept in mind and combine 2 or three different kits to achieve it for basic troop types. Remember for projects like this the Warstore (Battlewagon Bits) is your friend.
Do two or three proto-types and see what type of feedback you get, which ones do you enjoy painting the most, etc. to determine which way to design your army.
The other possibility is to look at other miniature ranges. There are some old warzone minis that would make very good nurgle traitor guard in my opinion.