It's not called green card in Poland. Rather there are work permits, residency cards or blue cards. This website has a lot of information concerning foreigners stay in Poland. There is residency permit for people opening business in Poland.
From what I remember you can get permit for stay in Poland if you open a limited liability company.
migrant is not for profit organization and you can call them or send an email if you have any questions.
In few EU countries you can simply buy a house to get permit to stay there. With permit to stay from other EU country you can freely travel to Poland (but not work here I think).
Are taxes really 40% of income after expenses?
The highest rate of income tax is 32%, but with costs of employer and obligatory insurances it's around 40-50% (en.wikipedia.org/wiki/Taxation_in_Poland). But I think you're interested with Corporate income tax and it is 19%: paiz.gov.pl/polish_law/taxation/cit