Do I need to get a PESEL number?
It's good to have one, is useful in several offices and other places in Poland. Besides why to ask, you should get it anyway. But I think without it is still possible to have a bank account though.. And yes, you need pesel first to obtain your NIP number.
Try accounting office "Rachel" in Krakow, they inform on their website:
We provide a contact in the following languages: English, French, Russian, Italian.
In Poland, what is the % rate between gross and net => I pay myself X, how much will I give to the tax system?
I'm not an expert, but I saw web calculations which estimated earnings on the level 75-80% after taxes and ZUS.
Corporate income tax is 19% in Poland - flat-rate.