Good to like Szczecin -- make sure to visit Gdansk too

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I don't think you would need any special permits to rent an apartment -- I think you can arrange payment in advance (let's say a week in advance) and you'll be fine.
I don't think the streets are very good so I'd suggest to buy a car (think about winter too

. I'm not sure about regulations -- but I'm almost positive they are the same as for all other EU citizens. The best would be to go to the city and find a large insurance location where you're likely to find a person who speaks English well and will find out.
Regarding credit cards, I think the banking system in Poland is quite well-developed. So you can open an account in Zlotych or Euro/USD; if you know Polish, you can use online banking too. Again, find a well-known bank in the city and you'll learn more.
Good luck!
Przemo