Good option if your rims is at least 7.5 inch wide.

It will make a noticeable improvement in comfort but not too much. For me personally 225/45 is the thinnest I'm still willing to use and I would never go thinner than that, so that's already the borderline for me. If you've tried 205/50 before in other cars, then the comfort is almost the same since they both have sidewall height which is just a little bit over 10cm. On the other hand 215/45 has only 9.7cm sidewall height (similar to 195/50), so it's about 0.5cm thinner profile.
As for performance, you would not notice any drop compared with the 215/45. I think it will actually be even better since it's wider and hence offers more grip.

Yes of course it will be a bit more expensive cause it's bigger.

You can try using used tires first so that if you're still not happy with it can resell without loosing much.