Authority site - a few questions when starting out

I've identified a niche that I'd like to build an authority site in - there appears to be low competition, and more importantly, I think, it's something I'm passionate about and it's something other people are interested in.

As far as branding goes, I've come up with a few names I like based on the keyword and all have the .com available - I've also checked for availability on Instagram, Twitter, and Facebook but not all of them are available, so I'm wondering: How important is it to have the name on social media? I know it can be an important element of brand recognition and driving traffic. One thing I'd considered was tacking on 'com' or 'dotcom' if the name is unavailable.

These are the names I've come up with. As far as branding, recognition, SEO, etc. goes is there any particular one which may be a better choice than the others?

<keyword>101.com
<keyword>hq.com
about<keyword>.com

Thanks!

Yes, just add dotcom or whatever to the end. It's going to be very rare everything is available today, unless you make up some unique word.