[DuskRaider]

Hmm... Well, according to what you're looking for, (piratical raiding force AND Undivided)... and picking the two best gods (Khorne and Nurgle), I'd say this, and mind you I don't have the Codex in front of me, so this is all off the top of my head:

HQ:
Chaos Lord, Mark of Khorne, Terminator Armor, Lightning Claws, Melta Bomb. This would be the ULTIMATE in Close Combat. (I personally am not impressed with the new Daemon Prince stats and lack of Armory options... Sv 3+ and no Termie armor option?! For shame!)

Elite:
Terminators! Terminators, Terminators, Terminators! I would also give these MoK, as the +1 T on a Terminator is pretty useless, IMO. I prefer maxing out a squad in most cases for the extra Reaper Autocannon, and giving the other Combi-Bolters. Deep strike when you feel the need, but a Land Raider is another nigh invincible option for moving these badboys into the thick of it.

As Stitch said, Dreads are pretty much worthless these days, which is really a pity... if you DO decide to use one, however... go with two CC arms. They're monsters on the charge and your troops have less a chance of dying.

Troops:
Now, here's the cool thing with the coming 5th Ed rules... You can use your Plague Marines to screen your Berserkers and improve their movement up the board. I myself am partial to Plague Marines, but I LOVE Berserkers in CC. Very little can beat these guys (Dark Eldar probably the only exception, IMO), but they suck in terms of ranged. So here's my suggestion. Max out your troop slots, but base it on your enemy more so then anything else. Not much a point in stocking up on a majority of Berserkers against a Tau ranged army, and Plague Marines will not be the best option against CC armies like DE, Genestealer lists, etc. etc. So do something like this:

Ranged List:
Plague Marine Squads, 4x 7 man:
-2x Plasma Guns
-Champion: Power Fist, Personal Icon (for those Deep Strike Termies)
Berserker Squads, 3x 10 man (I don't care about Sacred Numbers for Khorne):
-2x Plasma Pistols
-Champion: Power Sword, Plasma Pistol
I prefer a Power Sword due to the fact that A: You don't strike last, and B: It's just Khornate.

Melee List is similar, just make more squads of Berzerkers then PMs.

For Fast Attack, you expressed interest in making a piratical list... I'd think Bikers and Raptors would be GREAT for this. IMO, always... ALWAYS... give Chaos Bikers the Mark of Nurgle. T6 is awesome, that's better then a Daemon Prince, and when using that Turbo Booster you always have a 4+ Inv.
For Raptors, I suggest Mark of Khorne, give the Champion Twin Lightning Claws for a nasty assault monster, and 2 Melta Guns for Anti-Armor.

Concerning Heavy Attack, you could use a Land Raider as heavy armored transport, but that WOULD take up a slot. If you opt not to do this, I suggest a squad of Obliterators, as these guys can be adapted to fit almost any need, and are pretty much in my book, considered a 4th slot option for Terminators (quite similar, stat wise).
I always suggest a Defiler, because if anything it's a fire magnet, allowing the rest of your army to press on. It's great at wiping out vast amounts of infantry, and can tangle with the best of them.
Lastly, I say a Predator. These babies are great, adaptable, and I always bring one in ANY list I make. I always use mine entirely decked out in Lascannons... Defilers, Vindicators, and even Oblits can clear away infantry. My Predator is always solely dedicated to anti-armor, be it other tanks, large or synapse creatures, and scary opposing DPs or characters.

That just about does it. I'm sure some will disagree with what I've said (I'm sure some will think using all 3 Elite slots for Terminators), I use similar to this with my Death Guard army (with the exception of Fast Attack and Oblits/Berserkers), and haven't been beaten too much... Most of all, it makes for fun games!