is this 5461pln(net)deal a really special one? that i should never let it go away but TAKE IT?
No, this is not a CRAZY good offer.
On the other hand you can live a very good life for that amount of money in Walbryzch.
You should more consider if this is something that may be good for your career (which being stationed in a foreign country usually is) and also i can promise you that you will come to love life in Poland!
Its easy to meet new people here, and most likely you will find a beautiful polish girl and seatlle (happens to every expant in this country) so id def. say go for it.
But dont think that salary is a once in a lifetime.