since Typhus is a special character anyway, I would take him. you have to ask for your friends permission anyway.
I have a great unclean one behind me on the shelf, but didn't use him yet, so I can't give you advice on that. the Sorcerer Lord is a good idea. give him a pleage sword, a 2.
CC weapon and my favorite spell: gift of chaos! there's nothing like the look on your opponents face when you snot on his big boss.
you should take more marines with you. maybe add a havoc Squad. 4 Plasma Weapons work great against Carnifexes.
never can go wrong with a Cybot and a defiler is just too cool, to not use it.
never used Nurglings much, but Pleague Bearers are great, but size them down to seven models per Squad. it's just more fluffy