If you've got the witch king and some morgul knights, you need to get lots of orcs for them to lead.
Your best bet might be to get the new box set that's coming out, which will give you 24 morannon orcs, a troll and Gothmog, who would all add together with the first models for a nice Mordor army:
https://www.warhammer-community.com/202 ... osgiliath/That's coming out in December.
You could also watch out for the Mordor battleforce coming out at some point in next few months:
https://www.warhammer-community.com/202 ... ost-boxes/For a big game you'll want more than 24 orcs, so if you don't want to wait for these boxes to be available, just pick up a pack of 24 Mordor orcs or 24 morannon orcs as a first step. The morannons are stronger and probably a better choice for list-building as slightly better value in game points.
You might find morannon orcs really cheap on ebay as they had them included in the recent Pelennor boxed game and they'll also be in the new boxed game, so some people will likely sell the sprues off.
The box of warg riders could be a good investment, and watch out for if Khardush the firecaller becomes available, as it's a nice model and very good in the game.