It is very possible to build a site on an extension other than .com and have it be successful. Just know that in the future if you're startup get's lots of traffic the owner of the .com will receive some of the traffic and it may confuse your current customers. On top of that if you do decide to buy the .com at a later time the owner will know the value and have risen the price substantially. It's best to try and acquire the .com asap if you plan to buy it in the future.

There are many things to consider when choosing a name for your business, especially when it will be primarily online. You should always be building a long term brand so including a specific geolocation or marketing in the name will limit you to that area and industry. So you must consider if you will be offering services anywhere and if later you want to add web development or SEO. You're name should be ideally available in .com and available on any social networks you want to use. www.knowem.com is a good place to check. Make sure there are no trademarks currently on the name you choose. Make sure it's easy to say, spell, and remember. Ideally keep it less than two words and less than 18 characters. 10 more things to avoid when choose a name: https://nameniche.com/avoid-choosing-domain-name/. For a detailed and specific advice for your company feel free to schedule a call: https://clarity.fm/joshuardavis/free-consultation

Google has placed importance on responsive design because it improves user experience and therefore more users visit the site and stay on the site longer. In turn this raises the sites ranking in Google. Your site content still needs to be good for you to rank in Google. Making a poorly ranking site responsive will not see much benefit. Google has stated that it will be more favorable to responsive and user friendly sites than those that are not. Article: https://developers.google.com/webmasters/mobile-sites/ Site Test: https://search.google.com/search-console/mobile-friendly Feel free to message me if you have more questions.