1) Number of results in Google
2) Number of "allintitle" results in Google
3) Competition bar in keyword planner
Those metrics mean nothing!
What you should look? - These metrics suggestion are for begginers
You should check top10 results for these:
1) Page authority <25 (use Open Site Explorer)
2) Number of root domain backlinks + number of backlinks (same as above) <50
3) Citation & Trust flow <15 (use Majestic SEO)
When you should LOWER your criteria?
1) If OnPage SEO is bad (use Market Samurai for analysis)
2) If there are user generated content in top10 (yahoo answer, web2.0 blogs, article dirs)
3) If you find "niche websites" in top 10 (young sites with few articles targeting that keyword)
When you should INCREASE your criteria?
1) When you see more than 3-4 root domains ranking (exception is low quality niche website)
2) When one type of site is dominating serps (specially avoid e-commerce and news domination)
This guide will be enough for beginers to rank their keyword on #1 for the first time.
-Invest lot of time in keyword research. If you find a winner keyword keep researching to find better one and compare them at the end.
-If you are beginner targeting keywords with 1000-3000 monthly searches is fine
-Do tests - create 10 articles each one targeting different keyword and perform same link building tactic on each and see which one performs best. You will grow your SEO 6th sense that way