Like DIY, just the enclosure, or the whole kit and kaboodle?

In any event, I've used Synology and QNAP products for over a decade and both are solid. I've also built my own by just building a server in a full tower case and putting 8+ HDs in it.

It really depends on your needs and wants, but if you start looking through the two brands above, I'm sure you'll find something that fits your criteria.

Qnap or Synology. And with possibility to expand RAM and based on x86 compatible CPU.

Qnap is spyware, Synology not so much. If you don't #DIY your NAS, Synology will give you 10 years of reliable service before they no longer update security patches.