With your market samurai's in hand you do have a good beginning to seeing if a niche is at the basic level do able before you spend time and recourse's going further into the data of seo as mentioned earlier in the post.
Don't be overwhelmed it's not that bad 
The information for seo is a great checklist of activities necessary if you want to have a SEO'ed out site and there is help out there to help you decipher further if you want to go further at the level of "how competitive is it?" and can I compete
In the list above- authority sites, keyword in URL are easy to see at first glance when it comes to their backlinking or LINK LOVE (this is links pointing at the domain in question) there are some free and paid tools to help you to evaluate that.
Free:
SmartPageRank | Backlink PageRank Check - this tool shows basic results and basic top level anchor text (word string associated with the referring link)
Analyze Backlinks - shows anchor text, total links in (pointing to) and out bound links (pointing off page from the site in question) and it also shows page titles as well
These will help you understand the Link Love competition at a little deeper level.
As touched on above there are a lot more on-page factors if you were SEOing out your niches but at this point the information coming to us at 30DC is perfect for the phase of beginning dev. of the niches.
Actual on page elements where and when to put key terms & phrases and how will come after you have chosen picked your keyterms, phrases, and found a niche,
Structural elements such as H1 tags, where on page keyterms should be, underlining, and LSI'ed (related associated text that is within posts that you write and on keyword topic) happen after you have picked a niche and are beginning to build out your sites.
Hint want to see what competition is doing on page: browser plug in rec. in your browser set up - SEO turn the highlighting on- you will actually see where those keywords are on the page.
Keep in mind that a lot of the #DC's are just starting off in this adventure and to date the info presented and tools provided have jump started the non seo types years ahead of non-30DC (no IM /seo exp) .
A great position to be in in only 7 days!
