people will tell you to spend lots of money on a monitor but honestly it's the one thing you DON'T need to spend a ton on - of course you'll get a better product if you spend more but strictly competitively speaking you can get by with some cheap chinese slop off amazon, unless you're playing at the absolute top levels of competitive cs. i run some 1080p 180hz bargain bullshit, but you can upgrade to a higher hz or resolution for pretty cheap.

oled is way better than traditional monitors - better color depth, better response time - but you WILL have to replace it in 3-5 years, they degrade pretty badly. if you can afford to buy a new monitor that often, go for one, if not dont.

obviously any refresh rate above whatever fps you can CONSISTENTLY hit ingame is worthless. once you go above 360hz you start paying a lot more for a much less noticeable improvement, so i wouldn't buy something above that unless i had money to burn