I'm going to try to keep up the pace here. I might not be able to get a post done tomorrow as I've got a lot of work to do, but I'll try to get the last generic HQ choice done. So Dark Apostle today.
As a Word Bearers player I will almost certainly be taking one in my army, so I'll try not to destroy my opinion of them too much. The first thing that I noticed when I was looking at his entry isn't too exciting, it just specifies that he can take Veterans of the Long War for free. Why would you not do that? You might as well just auto include it in his Special Rules. Speaking of special rules, here is the first appearance of the USR, 'Zealot' giving the Apostle Hatred and Fearless, so already you can put him in a big squad and he'll buff it pretty well, a bit like a Marine Chaplain (no surprises at that comparison).
He can take Chaos ranged weapons, rewards and artifacts but honestly I wouldn't. He already comes stock with a Power Maul (thunder hammer), grenades and a 4++ save. He also gives all units within 6” his leadership and members of his squad get to re-roll on the boon table.
The only thing that I would be concerned about is his lack of attacks. He only has 2. So in every combat he should hit once, and reliably kill/wound (depending on toughness and eternal warrior) 1 model per turn of combat. So maybe a Mark of Khorne is in order. It's the cheapest of the Marks and it'll give him an extra attack enough of the time to make an impact. But then the question comes down to, do you really want him in combat, or just to buff a squad?
I'd say that the answer to this is yes. What good is buffing a squad if you're not going to do anything with it? I like rhetorics this month. The other way that I could see you using him is castling up your fire base and objective campers around him and making your Obliterators leadership 10 or Fearless depending upon whether you've joined the squad, then attatching him to a different unit to charge with/take a charge should the enemy get too close. But then are you getting enough use out of him?
I think that this is another thing that requires playtesting, and who knows? As I go through the book I may find units where I think, 'a Dark Apostle would be awesome in here'. As I do, I'll let you know.