I would drop the part about "if it works well it can become a paid role." This line is usually insincere.
If it's not extensive work, just pay for it. Treat your employees with respect. Unless you really are looking for cheap labor.
A month long internship with an unknown company/project is not going to be very appealing. If your project is famous and is going to look great on a resume, and you have amazing people and the intern is going to learn a lot from them, that's a different matter, but with what I see in your post, I don't think so.
If it's not extensive work, just pay for it. Treat your employees with respect. Unless you really are looking for cheap labor.
A month long internship with an unknown company/project is not going to be very appealing. If your project is famous and is going to look great on a resume, and you have amazing people and the intern is going to learn a lot from them, that's a different matter, but with what I see in your post, I don't think so.