What would be the minimum monthly searches a keyword should get to base the site off of it? I'm guessing around 50-100k with a lot of sub keywords at 5-10k for the articles but I'm not even close to being a pro at SEO...
Well it depends on the value that each visitor is likely to bring. If I had a site on loans for instance and could get just a few thousand uniques a month then I could probably make several thousand dollars a month which is more than enough to make the site worthwhile. However you have to take into consideration the competition for that keyword. Loans for instance would probably cost me more than the revenue mentioned above so it would in fact not me worth my time and effort. I base all my decisions on an ROI basis and am happy to build a site that only brings in $500 a month if it only costs me $50 to get that traffic/revenue. It's not an exact science but it works for me.
I agree with pedigreechump6. There is no specific minimum monthly search volume that you should be targeting. In some niches, you can still make good money without a lot of traffic. Ideally the niche you are targeting would have related keywords that you could target in the future.