I know you have your heart set on Nigerians, but have you given Nubians a thought? They are better brush clearers, produce more milk, and are more readily available than Nigerians. Their milk is 5% butterfat compared to Nigerian's 6%, and they produce over twice as much.
Another thing to take into consideration- you have 3.5 acres. I laugh at the thought of 2 nigerians trying to deforest 3.5 acres. The truth is, stuff will grow back faster than they can eat it. Nubians are triple the weight, triple the consumption. Plus, they are larger and have longer legs to climb through the brush and reach vines. Nigerians are tiny, short legged, and will probably not be able to do much compared to a Nubian.
In short? It's totally up to you. Nigerians are a great breed. I'm just trying to point out that Nubians probably would fit your requirements more than Nigerians.