Drupal developer since 2003 on hiatus.. How easy will it in the future to upgrade from Drupal 7 to Drupal 8?

I wanted to designate that I'm not a complete newbie, because I first started working with Drupal in 2003. I have developed a handful of sites over the years. But it has been almost four years since I have looked at Drupal or done any kind of web development!

But now I am back and I am about to embark on major web development because I'll be developing 10+ personal and business web sites for myself over the next 1 - 2 months. Thus, I am by no means an expert with Drupal, but I have the basic idea about Drupal from using it in the past.

I remember the old adage of Drupal: "When will the next major Drupal release happen? Answer: When it's READY!"

But I just have a question because I am trying to make plans. I want to start developing the 10+ web sites as soon as possible. Of course, I'd love to start right out of the box developing with Drupal 8, but of course it's not ready for production, and I'm not advanced enough to really help with coding or bug issues. Plus, with limited time I need to just focus on developing my sites and creating the planned content.

So my question is this.. it makes sense in my case (I think) to install the latest stable version of Drupal 7 and start developing these sites ASAP. I'm just concerned about how hard it will be to upgrade to Drupal 8 in a few months?

And also, it says Drupal 8 will be more mobile friendly. Of course, Google is moving in this direction in 2015 for SEO with ranking mobile friendly sites higher. Would it make sense to start developing on Drupal 8?

Or should I stick with my first idea and start developing ASAP with the latest stable version of Drupal 7 and then upgrade to Drupal 8 when it's ready? And will it be difficult to update a full production Drupal 7 site? And will I get the full mobile-friendly features of Drupal 8 if I upgrade?

Thank you for any insights and help with this! I need it!