If I wanted to start a Dark Powers of Dol Guldur army, what would you guys suggest? 500 points? 600 points?

Depends what you and your opponent want to play... Points wise. Do a list for both I guess.

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Can I jump in on this actually? I too am thinking of doing a dark powers list at some point, but centred on the nazgul and possibly the necromancer. How viable do you guys think a pure dol guldur nazgul list or possibly a mix of that and the necromancer could be? Say at 5-600?

I've actually picked up the models for this. I got the Dol Guldor Nazgul 2 and 2 boxes of Hunter Orcs. I plan to add Gundabad's later. I think that is a good (and fairly affordable) start.

I’ve used the dark powers a handful of times, with points varying between 500-800. They are a fun faction to play. I wouldn’t use the Necromancer in any game under 700 points. While he is a powerful model, he’s just too expensive to use in the circumstances. He starts to find his place in 750+ point games.

For starting a collection, I’d get two boxes of hunter Orcs, one warband of Gundabad Orcs, the Necromancer, and one of the Nazgul of dol guldor sets. I know that’s basicslly everything, but it’s all so good.

You can use them as a pure army in lower points games, but I’d consider allies too. In 750 and higher games, they are extremely powerful due to the Dark Powers army bonus. It turns the already powerful Necromancer into an absolutely insane caster.
But for me personally, if I’m not using him in the army, I’d ally the Dark Denizons of Mirkwood. It is both thematic and efficient. Here are my reasons.

Without the Necromancer, you’re Nazgul will have a harder time resurrecting, which means you’ll inevitably be conserving your might for the failed resurrection rolls. That’s a bad spot to be in. Having the ability to come back is a double edged sword. Sure your leader and killing machines could feasibly not die, but their might points will be held in reserve. That leads you to severely limiting your options. Let’s say one of your Nazgul is on his last point of might and you lose a key priority. Do you call the heroic move, or hold the might to keep him alive if you end up getting trapped? These are just things to think about.

That’s where using hunter orc captains and Gundabad orc captains can help. If you had one of them around they could be a cheap might caddy. If you ally the denizens of Mirkwood you get some neat options in the form of spiders and bat swarms. Even the spider queen.

The Dark powers have some solid troops, being the only army list that has access to both hunter Orcs and Gundabad Orcs. If found that using Gundabad Orcs with shields as the front rank helps block your vulnerable hunter Orcs from shooting. When the lines clash the hunter Orcs can flank the enemy and do what they do best.

Tl;dr. They are a solid faction at any points level. Don’t use the Necromancer under 700 points and try to utilize allies in smaller games. In 700+ games, use the Necromancer and no allies so you can benefit from their army bonus.
Hopefully you can find something helpful in my rambling.