Keyword research Questions

Hi there guys, i've been messing around with keyword research as my first attempt to rank dentists locally was a total failure. I've been reading a lot of threads here on keyword research and have some of the technical research down, however i have not fully understood the "is it a good decition?" and "why is it or why is not a good decition"

Hope you guys can help me with the following example i was trying just a few minutes ago.

Say i want to rank for this keyword: "best watches for cardio"

So i perform an intitle search on google to see if there is someone who has a site with that specific keyword: these are the top 5 results:

1.-http://www.wareable.com/fitness-trackers/best-heart-rate-monitor-and-watches
2.-http://www.wareable.com/fitness-trackers/the-best-gps-running-watches
3.-http://www.livescience.com/41769-best-heart-rate-monitor-watches.html
4.-http://www.digitalfundraisingawards.co.uk/insanity-workout-shaun-download/insanity-workout-legal-download/watch-insanity-plyometric-cardio-circuit-online
5.-http://f-m-s.com/langer/

From what i see, maybe only no.2 is related but so far no site has "best watches for cardio", in fact, from no.4 and so on, search terms are unrelated (no1 to no.3 are about watches though).

Here start my questions: is it a good idea to try to rank for that keyword? wareable and live science have really high authority but given the fact that they don't have the exact phrase neither in the title or url, is it a good decition to try to rank? why it is or why it isn't?

No.4 and 5 have no backlinks for that specific page but have about 17k backlinks for the entire domain, so my questions are about domain authority and page authority:

If the main domain have that insane amount of backlinks is it better to give up (low budget) even if they don't have On page SEO? When is it a good choice or when is it a bad choice?

Thank you guys

Here are some of the on page factors that statistically correlate with ranking position in Google for your specific example keyword:

1. Google Verify Meta Tag
2. Keyword matches in P tags
3. Page integrated for social sharing
4. Keyword matches in the first 100 words
5. Open Graph Type is article
6. # of facebook comments the URL has
7. Keyword matches in the Open Graph Title
8. # of Google plus ones
9. # of facebook shares

BHW wont let me upload the spreadsheet with the measurement data so a screenshot will have to suffice that I actually did measure and calculate the correlation.

Keyword research is one of the most important, valuable, and high return activities in the search marketing field. Ranking for the right keywords can make or break your website. Title tag is the most important on-page factor. Closer the keyword is to the beginning of the title tag.

validseo: Thank you very much, diggin in your profile i see your approach is more an accountable kind of scientific one. I can't fully understand the image you kindly posted but i learned a lot about other on page factors thanks to your post. Thank you very much and i'd love to hear more of your work.

clevelandslim: Yeah, that is what i heard in almost every thread on BHW and that's why i decided to spend at least 1 month just getting my keyword research down, slow but effective. Thank you very much.