almost all of them don't have built in hardware RAID. I don't really trust software RAID. Mostly rebuilding if the software crashes or my hardware crashes. Even if I was ok going with soft RAID
Most people here are the exact opposite of you and don't trust hardware RAID especially cheap implementations in a USB based DAS box. Software RAID is far more flexible and makes your setup independent of the hardware RAID cad dying.
A NAS is great when you have multiple simulataneous users. What kind of computer do you have? Do you have a desktop computer in an ordinary case? How many drives can it hold internally? If you've run out of space just buy a bigger case and move the motherboard etc to the new case and put the drives in the same case as the rest of your computer.