Because of the higher cost of living in Poland, especially for foreigners, and the fact that you have to pay for your own relocation, 15K would effectively be less than you are making in India, and 18K would be approximately the same as you are earning in India. In either case, there would be no point of moving to Poland.
There is lots about Katowice on this forum. It is considered the least attractive of the large cities in Poland. Most people consider it a gray, depressing and ugly industrial town with little charm, especially in the winter. Personally, I agree. About the only good thing about Katowice is that the cost of living is a little lower than in Warsaw, Kraków or Wroc³aw. But, in my opinion, doesn't make up for the lower quality of living, especially for your wife.
Like I said above, Poland is not an attractive place for Indians to work, except perhaps extremely experienced senior managers, administrators and consultants who can command extremely high wages. There is no "Indian Community" here like in London, Chicago or Sydney, for example. English is not widely spoken, especially by older and less educated Poles. It's more difficult and more expensive to live if you don't speak Polish. You can end up feeling socially isolated and excluded, especially your wife. Winter can be very depressing for people from warmer climes. Concentrate your efforts on finding a job in a richer country where savings potential is much higher and the quality of life is better than in Poland. And, like I said, rely on word of mouth rather than recruiters or the internet.