SEO: Focus on keywords

Hi Guys,
Being busy and all with the whole SEO market …what are the most important factors to choose your longtail keywords. High search volume is one of them, a bit of insight of the competition in Adwords, Search results and KEI …how do you realy choose the best keywords?
Does a keyword need to have a minimum search volume?
Is there a max search result for a keyword to choose it and score with it?
And what about the KEI? Important or sidedish?
Looking forward to your replies or step by…
SEO: Focus on keywords

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