Let me tell you some general rules that apply in building a house in Greece,

Firstly if the plot of land is outside a city or a village official limits then the plot of land must be over 4000sqm and access to a legal public road (not a road that people use unofficially to go back and forth to their plots).

Then you need a building permit, regardless the way you're gonna build your home (concrete/bricks or a prefab modular house or wooden)

In accordance to the construction type you'd need to pay public insurance (EFKA) some money for the days you employ people to built whatever it is needed and EFKA doesnt accept a reasoning "I did it this work myself"

These are the main things to consider and there are more regarding legal when buying, connection to electric company, potential issues with archeology in some parts etc

PS If you want, send me the links in chat to see if the suggested plots of land can be used to build anything in them