I cant help but think that 2k is a little too large for a tally list; beasts of nurgle really arent needed and are really only there because there simply isnt another decent nurgle unit you can take. If you can play it at 1500-1750 you'll have a much better army...
As for the list itself:
Ku'gath- almost an essential for a tally, a total waste if not playing a tally. GUO are simply better then him in al cases except where you are trying to get a few extra cheap kills early in the game. Luckily for you that is exactly what you want to do.
Epidemius- well yes, of course you need him
Beasts- almost identical to 2 plaguebearers except they arent troops. Slightly better in prolonged combats then multiple plaguebearers but can't score... so never take them instead of PBs, but in this case you dont have a lot of choice. Always give one noxious touch, allows for wound splitting and will increse the damage output of the model by 22%, a win on both counts (so unlike the PBs noxious touch is well worth it).
PBs- fine, I would take an icon or 2 and wouold want a 4th unit of PBs instead of a unit of nurglings but its not too important.
Nurglings- when you hit 10 on the tally these guys are nasty, when you hit 20 they are the most godawful evil things in the game. DS them in cover and keep them vaguely clear of the enemy early on, aiming to get into combat turn 4-5. The reason for aiming at cover is simple: while you might take 1-2 wounds from dangerous terrain you have plenty to spare... and then you'll have a 3+ cover save instead of their 5++ invulnerable (its massed fire that these guys dont like). If it gets really bad go to ground for a 2+ cover save. While I love them they are a little hard to use early in the game and cannot hold objectives, which is why I recommend the 4th unit of PBs, who are better earlier in the game (geting improved by each of the tally levels) and can hold objectives for you.
DPs- wow those are some expensive DPs, and yet they lack the vital upgrades. The best 2 upgrades for DPs are wings and cloud of flies. Wings allows you catch any vehicles that just try to drive away from you (certainly nothing else in a nurgle list can get them) and to make it into combat turn 2 (hopefully). CLoud of flies is very cheap and is awesome, giving you frag and defensive grenades, so you keep you high I when attacking and take the steam out of enemy counter-attacks. One upgrade that should certainly be cut is unholy might (unless you play a lot of land raiders), since noxious touch is always going to be better and against AV10-11 the +1S simply is not needed, and its only really against Land Raiders where its a real help.
Personally I would play DPs in one of 2 ways: either make them suicide princes (MoN, breathe of chaos... thats pretty much it) or make them more durable, I use a mix of winged and non-winged DPs, the basic guys have Iron Hide, cloud of flies and daemonic gaze, while the winged DP doesnt have the ironhide and gets noxious touch instead. The reasoning being that the winged DP hits closer and tries to survive through to charge whiel the others stand off a little, getting the tally up a few points with Ku'gath getting into combat after I've hit 10 kills so dont need either noxious touch or unholy might... they proved to be pretty good the other day, since one tooka charge from Abaddon and 4 termies and survived through a few turns as 12 plaguebearers, 2nd DP, 16 berzerkers and Kharn joined the fight.... in the end I lost the 2 DPs but they survived long enough to kill all the termies and berzerkers getting the tally over 20, leaving Abaddon and Kharn facing about 10 pissed off and fully upgraded PBs, with more plaguebearers close by and ku'gath and Epidemius with nurgling bodyguard just killing off the rest of his army and joining them- DPs are
very hard to get rid of once in combat
