I was just wondering, what units to include in a necron army that would be good
This.I am no expert so check out the necron army list thread. From what i have seen though there are some constants or regulars in the lists. Warriors, with only one troop choice that is fairly obvious. The monolith, being the hardest to kill tank in all of 40k. The third is destroyers. General opinion is that they are a very effective unit. The necron thread should tell you much more.