For Nurgles rot I have never played against or with CoC but from what I hear you can ether play with out it or offer a means for people to cure it (in the toughness roll for it or by paying a priest like the sawbones rules).
Possessed I like a lot.
My take on how to play them early is take the most common strat with them of try to rout early by sacrificing a henchmen. But what I like to do is take a warband of 4.
Magister, 2x Mutants (with one mutation each, I like Scorpion Tails), 1 Brotheren (unarmed).
Send the Brotheren out ahead to die with the others hidding and maybe trying to do some objective if able (like tresure hunt).
once the Brotheren goes down you run away.
The advantage to this is you save alot of GC for your possessed, you lose a low value henchmen, and you have a vary high chance of getting the underdog bonus from that first game with a WB rating of 40. That gives both your Mutants an advancment and also that Brotheren.
After that you can afford a Possessed (maybe both) and start to invest in your beastmen henchmen (who you want for your LGT). If you don't mind this strat for more then one game try to hire no new troops (other then replacing the Brotheren if he died with a Beastmen) and do it again. At most your WB rating is up to 48 well other warbands are going up over 100 at this point and thus another loss but again underdog bonus to get you much stronger Mutants and a lot more gold for your Possessed.