My firts PC build

Windows 10 is fine as long as you disable the telemetries and bloat. There are guides for this if you google about it.
You will want an xbox controller because most games work flawlessly with xinput and have very wonky dinput which means you need to download an xinput emulator. It isn't hard, but it's just extra hassle. Wired xbox 360 controllers are cheap and will most likely work for everything you want to play, the only problem is the dpad is shit. You can spend a bit more and get the xbox one s controller, and make sure it's the s version since the older ones have quality problems.
Yes, you can sell your old graphics card and update to a new one down the line for a quick and easy upgrade, cpu upgrades aren't essential and even if you do, you'd just need a new motherboard and a cpu, the rest can be reused as long as the ram is still ddr4.
Keyboard you can get whatever you want, just get what feels good. Google nkro/ghosting and figure out if it's something you need (to avoid) in a keyboard first though. Mouse I recommend you get a good one with accurate tracking and no angle snapping. High DPI is literally useless, it's used for marketing (it's a high number so it has to be good!).

Having built my first computer back in March I will endeavor to answer your questions to the best of my knowledge.

1. When installing windows 10 you have the option of turning off updates entirely. However this isn't recommended as updates are needed to fix and patch the system as they becoming apparent. You also have the choice of setting the time in which your computer can update without randomly doing it in the middle of the game or something. Like having it update at 3 am rather than 3 pm.
https://www.youtube.com/watch?v=LbpqkiaO7q4&t=1380s has some good tips. You'll just have to FF to the windows installation part.

2. http://www.ebay.com/itm/White-Wired-USB-Gamepad-Controller-Joystick-Joypad-For-Resembles-XBox-360-PC/381989031677?_trksid=p2485497.m4902.l9144 I got a blue one of these and it works just fine.

3. You'll need to get an Retail Key code for windows 10 rather than an OEM key. OEM keys are bound to your motherboard and some parts while Retail keys allow you to move your data from one computer to the next when upgrading or replacing parts. Just know that Retail keys cost more than OEM keys.

4. Not really. A membrane keyboard is just as good as a mechanical one in this situation. Plus it's quieter. The mouse isn't much different as a gaming mouse and a normal mouse operate the same.

>>341306
>turning off updates entirely
How the fuck does that work, first time I ever heard of it

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>>341305

Thank you so much, anon. Another friend recommeded me the Xbox 360 controller, so I might give it a chance.

>>341306

First of all, thank you so much. I am sorry, but I don't quite understand your third point. The thing with the keys is about the option to upgrade without losing data?

>>341310
do be watchful with 360 controllers, the dpads can be a bit shit. the xbox one controller's one is a bit better, but the 360 is more ergonomic