Is there a Microsoft Store? Yes, there is (but you don't really need it.)
And Will I get error messages or something like that? Yes. But checking the docs, asking in atlas discord or fourms can solve 99% of the problems you experince.
Is the system safe? Atlas is open-sourced. All files we use are on our public github repository. Atlas is based on the Microsoft Windows 10 (with unneeded microsoft stuff and bloatware removed), so as long as Windows is safe, Atlas is.
(added) if you are asking for anti-virus related "safety", please download anti-virus.
Is the system stable? Atlas is stable enough for gaming purposes. Even though Atlas can be used for work, i personally won't recommend it. If you need a system that is lightweight then normal Windows 10 and stable enough for work, i recommend Windows 10 LTSC.