I have not found a publicly available base theme using both Sass and Bootstrap for Magento 2 yet.
According to Alan Kent, Magento is planning to move to Sass in the future, but it hasn't happened yet. Currently this proposed move takes the form of a forum thread, and this base Magento 2 Sass theme from Snowdogs. This theme does not incorporate Bootstrap, however.
The Snowdogs Blank theme is pretty nice and I'm building on it, but I found their Frontools alternative build project complicated and too unwieldy for our purposes. I'm creating an extension that compiles Sassan extension that compiles Sass as part of the normal Magento 2 deploy process instead.
I am in the process of incorporating Bootstrap 3 Sass into a Magento 2 base theme, using the Snowdogs Blank Sass Theme as a starting point, but I'm a long way off from completing it.
In addition to the issues of re-stying everything (including changing the HTML DOM structure) to make the best use of Bootstrap's base components and grid, the Magento UI library has it's own set of Modal windows and other JS features that will take a significant amount of work to convert to be "full Bootstrap". I hope to eventually release a base theme when I get it there, but that's all the help I can be at the moment.
I have looked at some paid themes like Paco for inspiration on good ways to incorporate Bootstrap - but they use the LESS version of Bootstrap.