The biggest issue with the DK in my opinion is the fact that it gets very costly very quickly. All of the options are pretty good, you'll just be fighting the desire to take too many of them.
I've seen people have success with equipping a DK with just a teleporter. The stops the price from getting way out of hand and allows for some fast moving AT assault. The doomfist is great for pummeling vehicles.
The greatsword makes the DK a cc machine but, again, it costs points. In a dedicated cc build it's definitely worth it tho.
I'm not a big fan of the demonhammer. Either go for the doomfists to save points or choose the sword.
The HI is probably the best ranged option. The HP is nice because of the large blast, but, in my opinion, the HI gives you similar damage with a similar range only it auto hits, ignores cover, and costs less. The HP also can't pump out the high volume of light vehicle-popping shots which make the normal psycannon so attractive. Don't get the GP, the GKs have plenty of other options for S4 in their army.
My thoughts anyway.