NO MASQUE!!!!! even one as devoted to slaanesh as I would not take her as she's a kill point waiting to happen(though she makes a nice herald
lol)
I would count the
LOC as fateweaver and go for a fate crusher build as its pretty tasty
I'm a big fan of grinders but it seems that
DP's are a better way to go. that being said I would never take just the one as one with have a ton of antitank shots lined up for it, but I would say that 2 grinders and your nurgle prince would be good ^^
you need more seekers as, though I hate to say it, they get ripped a new one in units less than 10 strong so another box of them would be good

they make nice tank hunters and also are good for killing high T mosters like wraith lords of nidzillers so a unit or two would be very usefull
letters I'm pretty meh about as they show up and get shot or spend too long foot slogging so I would say invest in some plague bearers as they can camp OBJs and handle bolter shots better than others. nettes are good in large numbers so I would go with a 20 wo/man/thing blob to camp/kill
khorne *spits* chariouts are very, VERY good (ugh, now I feel dirty :( ) and I would consider making/buying some fiends to replace the crushers and use the crushers to make into heralds on chariots as they mow stuff down pretty quickly
hope that helps