I want a Prebuilt NAS for Plex what is the best option I want to be expandable.
IMO the best budget selection for a "NAS" that is going to use Plex...it's probably the old system sitting in your closet.
Plex has it's own requirements so much as CPU strength to have it run smoothly and properly. That can be as low as something like a (modern) Pentium, maybe an older generation Ryzen 3 level. My own experience with the modern Athlon with Plex was less than desirable.
Motherboard with a few SATA ports or an unused PCI slot and a case that can hold drives and a fan or two.
W10/FreeNAS/Linux (supported) and you are in buisness.
The only advantage that I have found in regard to my somewhat limited experience with purpose built 'NAS' units have been power and heat footprint. I currently have a Synology that I am not crazy about in spite of it working fine for it's purpose. I have used a "PC" build on FreeNAS and wasn't too crazy about it. My so far favorite for use is W10 due to my familiarity with it and prefer to use Pro such that I can readily remote into it for administration without having to have a permanent M/M/K.
Most importantly for my use is that when you use FreeNAS or Synology they are using a disk format that cannot be plopped into another PC and read. This obviously has things to do with the format you use (RAID and so on), but utilizing W10 and a variety of formats, the drives can be put into any other Windows PC, initialized, and read.