Okey guys, thank you for all our answers. Yes i notice that i choose wrong RAM after i post this post.
So if i will go with DDR4 (as it should be of course), change power supply to a better ones. Will it work fine?
To connect power supplies i already have an idea how to.
Risers from aliexpress are good, but it's ~3 weeks of waiting + Chinese new year so it can go up to 4-5 weeks i guess. Here in my country there are plenty of resellers who bought at ali and adds just 2-3$ profit. so i can buy on a little higher price but in 2-3hours.
Indeed, risers from China will take much more time to arrive. For the PSU i bough a server PSU to power all GPU and a smaller (and cheaper PSU) to power the motherboard because high power (non server) PSU are very expensive.