Winters can be very cold but cities are still recommended even in the winter. Insurance still recommended for accidents, luggage and personal belongings. Your passport should have at least 6 months validity left. Most major cards are accepted in hotels and restaurants and shops. Travelling in Poland Communications are well developed particularly the public sector.
Trains are excellent, cheap and frequent between the cities.
Buses are very cheap and plentiful, but you need to know where you are going. Taxis are everywhere and reasonable. Watch your charges at airports in particular. Shuttle buses are operated in all international airports and city centres are a fraction of any other costs. Driving Roads are improving all the time.