Level of English for work

   As far as I understand, the system here is that the higher the level, the better. But maybe there is some lower limit that I need to reach exactly.

That's for sure, business English is still different from spoken English and so on. To begin with, all you need to do is find a good tutor and discuss your strategy and learning goals with him so that he can select the best curriculum for you. Promova has different programs for both upper intermediate english and higher levels. Additionally, there are a lot of experienced teachers out there, so I recommend at least learning more about this platform.