You first need to read about the restrictions on buying farmland - which apply to both Poles and foreigners. Basically you need to be a registered farmer.
For your plans you might like to buy a small plot next to a lake where it would be easier to get permission, but you will also probably have some neighbors. A wooden house is just for summer ? Winters are going to be a big problem if you want to live their year round so make sure the road is passable and you have a water supply.
It doesn`t need to be a farmland. There are also construction/building plots, excluded from farming.
Where to start? Start by checking ads on major buy/sell property sites. You can also contact an estate agency in a big city but those in small ones could be also appropriate for your needs.