If you want a NAS on the cheap my preference is just get any cheap "normal" PC, a case with a good amount of HDD bays. Move the drives into the PC, and you have all the expand ability you could dream of. You can find plenty of DDR4 machines for cheap now. Then as ram prices come down you can go up to 128gb of ram as long as your board has 4 slots.
Anything on craigslist/FB marketplace will work.