Whether a treadmill is worth it really depends on how well you know yourself. I see this from a different angle than a lot of people: as someone who regularly works with people in my job who don't stick to their fitness goals, I can tell you that the motivation for indoor training is completely different than outdoors. The monotonous feeling that others have mentioned is real - running in place just feels different than feeling the ground under your feet and having your surroundings change. But there's a trick that hardly anyone does: put the treadmill in front of a TV or your favorite shows, and actually watch it engagingly. Not half-paying attention, but really. If you're watching a series you actually like, half an hour flies by, and you don't even notice you're doing monotonous training. Some people use podcasts or audiobooks too - that works just as well.
The second problem is space. Before you buy, you need to be honest with yourself: do you really have a place where it won't be in the way and where you can see it regularly? If it disappears into a corner, it becomes a dust collector. A smaller, foldable model might be more practical for you than a big machine. And about the price: you don't have to spend 3000 euros right away. There are solid treadmills for 600 to 800 euros that last a good while. Start with that and try it out to see if you can live with it. If you're still running on it after three months, then it was the right decision.