How do I find out if a property is for sale?

Otherwise, ask at the land registry office.

Just send an email there & they will reply to me? How do I provide the coordinates – via Google Maps coordinates? Isn't something like that stated in the land register?

And who tells me if there is an obligation to develop the property there?

Ask in your local building authority office. They can tell you the building obligation and very often know who owns which property. Otherwise, you have to make a request at the land registry office - for that you need the plot number - the building authority of the municipality can tell you that. For us, an email to the land registry office (district office) is enough - it costs about 30€.