how do you decide what keyword to target?

there is a keyword that has just under 400,000 competition in quotes on google, is it worth targeting?

How long will I expect my youtube video to rank on page one, or how many backlink services do I need to buy to beat that?

It will most likely be quite hard to rank for such a keyword. Check the top 10 in Google for your specific keyword and see how well they optimized their sites/videos.
Try and aim for a keyword with less results and especially less competition first