I'm handcrafting an extremely simple wordpress site, with minimum features, functions, templates etc. Is there a default style.css that I can use as starter? I have found this one but it is quite old - refers to WordPress 2.6. Should I use this?
You can certainly use it, but it is not going to cover everything. Plus a lot of the style is going to depend on how you structure your content and other such items like your menus.
If you are looking for just a blank stylesheet that contains all the default classes and such that WordPress uses in outputting the content, there is an article on CCS Tricks that has a pretty comprehensive list.
One thing I would advise is learning SASS to help you write your CSS faster and also develop a workflow that can be ported from site to site. It also has some nice time savers like calculations and variables (e.g. $baseColor: #0000ff; background-color: $baseColor)
CSS and HTML came a very long way since Wordpress 2.6, so did Wordpress itself. The file that you are linking to is in all propability useless now due to a thousand changes over the years since the years of Wordpress 2.6.
There a quite a lot of minimalistic starter themes available which you can use to build a theme on which will actually be faster than building a teme from scratch. IMHO, it is quite insane to start a theme from absolute scratch, but that is just my opinion :-)