Hi there,
I would definitely be looking at the Moz Keyword Difficulty tool and other tools like SEMRush http://www.semrush.com/ both are great for gaining competitive insight.
Your second question is well if they are competitors then surely you would be wanting to at the very least have a competitive positioning of your online content with them. I have different competitors for instance for different keywords, I have common grouped competitors and niche competitors, they may be domestic, foreign, a specific vertical and so on.
If you were starting out on employing an SEO strategy then your question actually might be construed as a little back to front, as I would be wanting to analyse my own site's keyword phrase conversions to determine which are the most effective, have the highest CTR etc. and then utilising tools such as the ones mentioned to see who am I competing against with my best converting KWs. Plus you would be able to identify any mismatches of who you deem as your natural competitors against ones that you are not aware of from the analysis you carry out on your own site traffic data.
Hope that helps a little.
David