Display Enlightment by NCX

I was going to ask if a high contrast would be relevant to my needs but if high brightness increases BLB i guess i don't need higher brightness that much.

The higher the contrast the better, assuming the panel has uniform brightness and color distribution rather than just a high contrast center which is common.  AHVA/IPS/PLS with >1200:1 are very rare, and the same two panels may (less likely now, but used to be expected from pre 2012 panels) have up to a 40% contrast ratio variance.  Reduced glow and uniformity are superior to a high contrast panel with more glow and back-light bleed.

What rise + fall response time will avoid ghosting and is good for casual gaming?

Oscilloscope measurements and photos can prove all 60hz LCD panels, many CRT's and plasma's suffer from ghosting (color streaking for fast pixel response times and smearing for long pixel reaction times) or phosphor trailing (CRT and plasma) which must not be confused with sample and hold motion blur which both LCD and OLED suffer from, and is reduced by increasing the refresh rate (60 to 75 to 120hz, ect). 

Almost all AHVA/IPS/PLS panels are fast enough for the majority (which you may not be part of)...those who are not using a high refresh rate CRT or overshoot ghosting and PWM blur free TN.

Is 18,4 ms good enough on a 75Hz monitor? What about 14ms on a 60Hz?

I can't answer this for you, but can guess YES if you're not using a PWM blur free TN without overshoot.

My monitor (Qnix QX2710) versus the Acer X27 whose response times you can look up here.  I also use an HP 24 Envy which is very similar to the AOC and significantly faster than my Qnix.  I always notice the response time flaws, including flaws from the fastest TN panels, but I can't stand using them, which is a common opinion from AHVA/IPS/PLS panel owners.

Is it a bad idea to use the AOC 24V2Q for gaming or is it good enough?

PC Lab PL measured 7.5ms for the TN panel in the BenQ GL2580...which is a Far Cry off the claimed 1ms response time.  I think the AOC is more than good enough, but if you want a significantly faster monitor buy a TN.

Should i get a second hand Samsung S24E650PL?

How much is a new Samsung versus AOC?  If PC gaming with a Free-Sync compatible graphics card the AOC is probably the better choice since it supports Free-Sync and 75hz, though the Samsung can probably overclocked to 71-75hz, has a height adjust-able stand, better image quality and is faster.  I can not recommend second hand monitors without knowing where they come from and what condition they come in.

What's SDR?

Standard Dynamic Range (HDTV/REC 709 and sRGB color space which are extremely similar and the color spaces most content uses) versus High Dynamic Range/HDR which is a form of wide gamut color quite different to the Adobe RGB standard.

fix the high response time on the AOC 24V2Q with the overdrive options?

Don't know, and the AOC response times are normal for AHVA/IPS/PLS.  Compare Playwares AOC measurements to a few others to be sure.

Are videos/movies/shows affected by ghosting, inverse ghosting, overshoot and other similar issues?

Everything is affected....but will most people notice?  No, especially in the case of the AOC as long as the overdrive is not set to the highest overshoot ghosting inducing setting.