Georgije

Your Windows 10 key won't work for Windows 7 unless it was originally a Windows 7 key that you used for Windows 10.
 
It works the other way: Windows 7 keys can be used for both Windows 7 and 10. But, it doesn't work in reverse with an original Windows 10 key.
 
You can buy a Windows 7 key for cheap through eBay, reddit, Craigslist, and other places.

My local retailer.
16gb one is on sale from 45$ and 32gb is for 40$ from 75$.

i would link it to you but they don't ship international

Probably because the price ratio between desktops is like 1/3 ( i mean to say that price  of laptop with same specs as budget gaming setup is 3 x more expensive ). The upgrading is also the point which is making a desktop PC a winner. On laptop in you can only change storage and RAM, maybe in certain cases you can add eGPU and change the CPU, and that is all. But in case with PCs there is wide variety of upgrades that you can manually do without a technician (and it is easier to do it on PC than on laptop).The category in which laptop takes a win is portability. I personally use budget lenovo laptop G50-45 (amd e1 - 1.86 Ghz, amd radeon R2 1gb) - nothing special. I use it for college. I don't have PC. This laptop basically limits my gaming "obsession" so i don't get distracted to often. Tbh i was never thinking about the latency between the mouse/screen, i don't think it has to do anything with  computer , only with uncalibrated drivers (GPU,IO ports). That is just my guess.