I would spend as much as you can afford on a new NAS. I purchased a TS-873A (slightly more powerful than the TS-464 @Becker2020 suggested) about a year ago. I wish the TS-873A was more powerful. If you are running QVR Surveillance, Plex, etc, you need a more powerful NAS.
I don’t know your budget - only you do. But get the most expensive NAS you can afford. I would stay away from ARM based units. Buy an x86 NAS.
Sometimes you can find great deals on a used NAS on eBay. I bought a TVS-672XT with drives for $400. The drives alone were worth more than that. I just recently had to have it repaired due to an Intel chip issue with that series, but it was still worth it. Now I’ve upgraded the processor to an i7 and it makes all the difference in the world.
If you want to just share files and do basic NAS functions, the low-end Celeron models are fine. If you want to do more complex things, you need more power.