Hello.
I don't post very often on this forum, as most of the stuff is not related to my country's reality. But as I saw that you want to invest in Poland, I decided to share my knowledge, as I am a Polish citizen.
To start with, 6,1% return in Warsaw seems like a good deal, as the properties in Warsaw are, on average, more expensive than in the rest of our country. But that's the information I read in some Real Estate journals and got from propert manager and I am sure that one can find better deals there. However, one can get, on average, higher returns in other parts of Poland.
I totally agree with maizeman about the possible risks. According to me, the possible changes of exchange rate is the biggest of them.
As I've written, properties in Warsaw are rather expensive, so they don't have that much potential to get more expensive. However, one cannot forsee the future and I am not any different.
But there are some other advantages of investing in Poland, not stated by maizeman.
It is highly tax-effective. One can pay 0 in income taxes, as when the building is older than 5 years, it can be amortized at 10%/year max so taxable income will be 0. In your case, you could amortize the property at 1,5%/year in the first 5 years, than change it to 10%/year.
One pay almost 0 in property taxes (it is something about 0,1%, but it's different in every city)
My parents have 5 condos as investment properties and 2 of them are rented with property manager. The PM charge 11% of rent for the service. I consider it rather cheap.
How many units would you like to build? How many rooms would they have and what would be the size of each condo? You could contact with a property manager and ask him about the rent level in Wesoła.
But, as I said, we don't have any properties in Warsaw and I have no idea about the local real estate market and I don't know whether the location is desirable.
By the way, you would be consider rich in Poland both in terms of income and wealth.