1. For salaries look here: tiny.cc/outsourcing-pdfand there tiny.cc/hays-pdf.
2. Between cities above 300 000 citizens renting prices and transport costs varies up to 100% (meaning Warsaw the most expensive and perhaps Białystok the cheapest). The rest is the same. Use numbeo.com to compare.
3. 4500 PLN is a minimum? yes. 4500net.
4. read this topic: https://polishforums.com/life/poland-quality-major-cities-cost-70877/.
I have written there about air pollution in Polish cities. Basically it's all ok, except of Kraków in winter. Trains in Poland are quite slow (3h for 120km), especially now, when many major lines are being renovated. Road network is underdeveloped when comparing to western countries too. Transportation in cities is quite good. Poland is very save and has average level of corruption. Quality of life is good if you earn enough and don't have to commute too much.
5. yes, as you would not be able to communicate with most of people over 35 - 40 years old.