Land in this area is roughly €1/m2, although it will be more if it has a house on it or existing amenities.

Regarding internet access, you can use our coverage checker :-) gardunha.net. We only cover a small area of Portugal.

There are some Facebook groups for people who have done similar things, although they do have some odd people in them. Lookup Pure Portugal.

"Is 15000 sqmt for 100k possible?"

Surely it is.

https://www.idealista.pt/en/point/comprar-terrenos/39.62296/...