Do NOT buy the Mordor Sourcebook. The new sourcebooks will be coming out in just a month or two with the new rules. Some years ago, I wasted too much money buying rulebooks just before they became obsolete.
General thoughts on Mordor: good, versatile army. High Defense (6+) will keep your troops alive, and high Fight Value (4+) will win them fights.
Orcs have neither, but make cheap spear support.
Black Numenorians have both, plus Terror. They are your best shield wall unit.
Black Guards have both plus Strength 5, but are too pricey per points...maybe worth it against Dwarves .
Morannon Orcs have high Defense, but bad Fight Value (3). With Strength 4 and spear, they make effective but pricey spear support.
Spectres can make an enemy Monster useless (as they generally have low Courage), so I'd recommend one or two.
Fell Beast is worth it for your Witch King. I would stay with Mordor Nazguls, as they are the best magic casters in the game. Dol Guldur Nazguls are great fighters & hard to kill...but magic is more usefull, and you already have great killers in this list.
Shagrat, War Leader, for example.
Grisnack & Gorbag are the cheapest 3 Might leaders in the game, I think. Generally, whoever has the most Might in the last few rounds will win.
Here is a handy Armybuilder for Excel:
http://www.lonelyknight.0fees.net/?i=1Here is a decent 600 point army with what you have:
Witch King on Horse 3 might 20 will 3 fate (get a Fell Beast)
6 Orcs with Shield & Sprear
5 Orcs with Spear
1 Mordor Troll
Mouth of Sauron Mounted
6 Orcs with Sheild & Spear
6 Orcs with Spear
Orc Captain (get Gris or Gor: cheap on Ebay)
3 Orc Shield & Spear
3 Orc Shield
6 Orc Spear
7 Might 39 Units Not super competative, but will do.
Or...trim out horses, Troll, and WK's Will points to 15 and get another Warband...48 Orcs with Shield & Spear should make your opponents nervous.
Enjoy.
With the Armybuilder, you can juggle your options with Mordor Uruk-hai, Spectres, Fell Beast, etc.