Can I see an estimate or a guess-timate for a WordPress 5.0 release?
5.0 won't be released until Gutenberg is done, and "this year" is as specific as anyone's gotten for that. At some stage it was supposed to be early this year, but it's already April and there's still a fair bit to go.
The FAQ states:
We are hoping that Gutenberg will be sufficiently polished, tested,
iterated, and proven enough to be merged into WordPress 5.0, with an
estimated release date of 2018.
You can also keep an eye on progress on GitHub.
Is it for certain that Gutenberg is the new default editer? And have
the development team said anything about, what is going to happen with
TinyMCE? ... Ideally, then I'm looking for official WordPress
It will be the default editor. There's a page on wordpress.org about it and the FAQ.
If I were to install the Gutenberg plugin today. What happens then
if/when Gutenberg becomes part of core? I hope/assume that they've
considered it, - but I'm afraid of making a solution for a client
based on, that I hope someone considered something.
Gutenberg isn't finished, or even feature complete. The plugin is available for testing, and some people are already building products based on it, but they will be following development and ensuring that it maintains compatibility as Gutenberg develops and eventually merges into Core. If you build a solution on top of it now, I would plan for future updates to ensure it continues to work.
A plugin is already available that will disable Gutenberg and keep the current editor around.
Personally, I would not build a client project on top of Gutenberg until it's in Core. Something personal, maybe, but not something where the immediate future has so much uncertainty. If I were developing a public theme or plugin, I would be working on compatibility now, but client projects are different and probably won't be getting the ongoing attention needed to deal with what is essentially still beta software.
It's also worth noting that Advanced Custom Fields plans to support Gutenberg, so this isn't an entirely either-or question to begin with. ACF won't stop working when 5.0 is released.