How is the living standard in Poznan? Does 5200 PLN enough for a decent living?

I think so :) :)

If you're still wondering.
5200 is a good salary, especially for someone without much experience. It should be enough to live decently.
What kind of job it is? If it has something to do with IT, I wouldn't say it's not probable.
There is a new school of digital art etc. starting up in Poznań, so it sounds pretty legit. I guess they would offer a foreigner a higher salary if they can't find anybody from here to do that job.

Renting a room from an English speaking landlord? A bad idea if they offer you such unfair prices. Just ask some Pole who can speak English to do you a favour and speak with a landlord in your behalf. I can help you if you decided to take that job and you still need help. You can also ask the company who wants to hire you for help with renting a flat or a room.