Honestly stick the chaos lord on the bike give him black mace and he will be a meat grinder in combat.
Sorcerer of nurgle
lvl 2 will have to take one of the nurgle powers which aren't really great. But regardless I would either take him off the bike and stick him with a squad of plasma plague marines (instead of the chosen) OR stick him with the chosen and try biomancy them into being more survivable. Or even the havocs?
Plague marines with plasma are fantastic so try get them in if you can.
I know what it's like though starting off a list and having your models there and people are telling you to use loads of models you don't have so just have fun
Your list is fairly decent for a 1000 point battle just be very careful that your Chosen don't get killed early in the game as they would be a big part of your offensive gone and over a quarter of your points gone. They really are not that survivable.