For 1000 pts, a block of Marauders is good. A couple of units of warhounds will shield your knights. You should have a few points spare. I'd suggest looking into either Dragon Ogres (and taking points from the Knights - Dragon Ogres are better than Knights in smaller games, despite the less number of attacks), or Spawn.
For 2000 pts, it's more open ended. You can either go for Monsters, Marauders, Chaos Armour. I think choosing one and running with it is safer than mixing.
Chaos units, due to their strength and number of attacks are capable of taking on anything - 100 Marauders with a Mix of Great Weapons/Flails and Hand Weapon and Shield combo's can go head to head with all but the very top elite (High Elf elites, Chosen Chaos, Grave Guard), and they're in more limited numbers.
Personally, I like Monsters, as you can get a good few units in, but the models (particularly Dragon Ogres) are generally bad - but Trolls are good, if you have Throgg in the army.
EDIT - Lances for Counter Heavy Cavalry, Sorcerous Weapons for Unit Munching. So with Knights being Knights, unless Khornate, I'd always give them Lances, and use them to munch through units, and save the Khorne Knights (16 Strength 6 Attacks will hurt the Heavy Cavalry at lot) for Cavalry Counter.
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