Thank you for your answers. I was first thinking about the R-25C, but have then been told by various people that I should buy something better. 250/440/450C has been mentioned, and also RC-62 and 64. The reason I am a bit focused on 64 is because I watch a lot of movies/tv-series and I want the best center for that (but of course not a center that "destroys" the other speakers) and because I saw someone selling the 64 used (price is now lowered to 500 dollars which is what I've seen 450C goes for used).
I will probably be upgrading the other speakers one day, but since I bought the speakers and receiver just half a year ago - it will probably be at least 2 or 3 years before I buy something better.
I really have a hard time deciding seeing some recommending to buy and others saying not to buy I guess I'll just have to think a bit more about it. I appreciate every feedback I get here, and please add more if you got other recommendations about what to do in this case.