You can build as many houses as you like your still getting the cost of living rent that's applicable to the country
I don't see Poland being a migrated to country from other eu country's any time soon
Best scenario us say you have six properties you rent out all to good tenants you get a steady reasonable income by no means lavish but money for nothing
London for example your laughing if you owned multiple property for rental demand will always outstrip supply extortionate rent, for some inane reason London's a hub people visit, live and work and seemingly want to
Poland is lovely but I don't ever see it being a hub for commerce or working migration etc
If everyone us building property for commercial reason it's highly possible to outstrip to demand
So you could end up with multiple property money pits
I don't have to pay a rent in Poland because i own a house (no mortgage or loan - so bankers can fuck off), there is no Council Tax in Poland but there is a Property Tax (less than 100GBP per year in my case). Basically i pay utility bills only. Tomorrow i can come back home and pick up a job for 300-400GBP per month and i will be in better situation then you or many other Londoners - because i own a house and i don't have to worry about money to pay mortgage/rent. That's all.
Migration to Poland - well you are wrong. It does exist. Poland is a favourite destination for Italian and Scandinavian people. My mum's friend is with Italian man for example. And do you know why he moved from Italy to Poland? Answer: hi is fed up with things in Italy (read: blacks and muslims everywhere). North Poland and it's sea side is packed with Swedish people. The same reason - no room for white people in Sweden soon. But you are left wing do-gooder so you do not understand what i'm writing about.
Property money pits - well once again you are wrong. In 2013 my sister sold one of my plots of land without any problems. A young Polish girl from USA decided to buy it (milions and milions of Polish people work around the World as you know). I did earn good money. And at this moment everything tells properties in Poland (especially land) will be getting more and more expensive.
My landlord keeps asking me about flat prices in Krakow - he would like to buy one and rent to British students (plenty of them in Krakow studying at Jagiellonian University - one of the best universities in Europe).
So i do not worry about my "business".