Could anyone help me out with some questions about PC building?

Its my first time doing this and I don't want to fuck it up. I gathered some info and watched a tutorial, but I still have some questions.

1. About the OS. The easy thing would be going for a Windows 10, but I've heard it can be terrible, rebooting the system randomly for updates and some other stuff. Is it still like this? Can it be avoided somehow? If not, would you recommend another OS?

2. Any recommendations for a cheap-ish controller? Or any kind of controller that might be good to check, in your opinion.

3. The setup is supposed to be easily upgradeable, right? I don't mind spending 1000€ on a machine if I can make it work for years.

4. If I am expecting to play mainly casually with the potential option of trying to get somewhat competive, does it matter much my choice on keyboard and mouse?

Thanks so much in advance. I know this stuff is actually pretty basic or it's more up to a personal opinion, but bear with me.