When finding keywords you pick them on the monthly searches they receive and the number of competition in Google. However when doing this what number should I go in for example keyword with no less then 100 searches each month and less then 100,000 competition?
You should not look at the number of search results to determine competition. A better idea would be to look at the pages which are featured on the first page as these are the sites you are competing with for the top spots, and see how much authority they have.
I don't have a minimum number of searches in mind when looking for good keywords. Whatever keyword phrase is not competitive and gets a decent number of searches is worthwhile targeting.
100 visitors/month is good. Target 100 of these long tail keywords and that's an extra 10k visitors per month. The great thing is even though you target one keyword in the article your site will be found for other related keywords, so it's even more visitors.