I find that Khamul is a good, well themed (pretty much "best themed" for a Nazgul) addition to a Dol Guldur list and is a good stand in while you're building up a large enough force to justify the Necromancer.
I think GW's biggest fail with DG is not including spirits in the base list. Necromancer??
So I personally like to ally them in as best as I can. It's easier in WotR than SBG. I'm not sure which of the two you are building (sorry if I'm over looking something obvious).Orcs are a great mainstay of any Mordor based army and will give you some good model count. I also find Wild Wargs very effective in SBG. For a cheap (points and dollars) start get each of those in numbers, with a couple Giant Spiders and one Bat Swarm. Use some generic Captains until/unless you want a Nazgul then drop in Khamul (on horse if possible...he can make better use of his special ability as well as be more effective). The Wild Wargs, Spiders and Bats give good mobility either for early engagement, objective rush or flanking while you move your main Orc force up.
The Mountains of Mirkwood are as likely home for any Goblin force as Misty Mountains so feel free to pull in allies from the MM list and stay in theme, and a large force of Easterlings was on its way to reinforce Dol Guldur during the later stages of the War of the Ring so as long as they're legal (I believe they are) you can use some of them for extra defense and strength.
I'm not too happy w/ the Spider Queen model either but saw an article on GWs site from a while back of chopping an SQ and Shelob and building two new models with the parts and have been working on that. They aren't easy but look good.