You normally have to buy minecraft first. There are cracked clients but those wont work with normal servers. I think there is a way to allow them in server settings but it means player skins wont be loaded and everyone would have the same skin. Idk if there are any other differences.
Awesome a will have a look into it. I chose minehut so I didnt have to host the server on my pc. I will check out the server software as well since classicube looks interesting.