I've been offered a job with ~ 12K PLN Gross in Wroclaw, and i need your input if this figure is going to provide a good life style to a family of 4 members?
That's great for a single person, pretty good for a couple, but really not very tempting for someone with two kids for whom they will have to pay for private English-language daycare and education. Taking into account travel and moving expenses and visa fees, it may not be worth the move, especially considering the fact that, even under the best of circumstances, you would not be able to save up a substantial amount of money in absolute dollars or Euros.
Also, i don't know if it's possible to get a job for my wife(a doctor with PHD) and what should be the steps to enter the market (some type of getting a certificate equivalence).
For doctors who do not own their own practice, wages in Poland are abysmally low. The wages for academics are also abysmally low. It is unlikely that going through the harrowing nostrification process would be worth the effort (that's the process of getting a certificate equivalence). It's safer to assume that your wife will not find work than to bet the farm on the odd chance that she might. Poland is a country that doctors leave in droves to earn better wages under better conditions elsewhere, not the other way around.
By the way, I'm a pathologist with a Ph.D., and I earn a lot more as a scientific translator and freelance academic advisor and mentor than I ever could as a physician or academic. After I arrived in Wrocław, I was interviewed for a position at the university here, and everything was just fine until I heard their offer, which was so low that I couldn't possibly accept it. There wasn't even any point in negotiating with such a low-ball offer. Wages for hospital staff doctors are so low that I wouldn't even bother getting out of bed for an interview in the first place, never mind go through the nostrification process.
Bottom line: you're better off either looking for a job in a Western European country like Norway or Sweden, in an English-speaking country, or biting the bullet and signing a five-year contract for a high-paying job in Saudi Arabia or the Gulf states, leaving your family behind. Sure, it's like going to jail for five years, except at the end you have a pretty nice wad of cash for your kids' college funds. Actually, I regret somewhat that I didn't do that myself.