Yes.Is 2k vs 4k noticeable in 27 inch.
4k will look much sharper but having said that I watch lot of 4k videos on my 2k monitor and it is still very impressive.
Yes you can change the resolution. My monitor is 2k but i play many games on full HD .Also, if my gpu cant handle 4k gaming, can i use the 4k monitor at 1440p without any issue?
My advice is this....if you getting really good deal on 2k but have specs that does not match 4k ...stick to 2k for now and later sell it when you have good 4k specs.
Monitors with good HDR support and OLED will see price becoming more affordable sooner or later.