Those HP, IIRC, only use (SFF) 2.5 inch drives, so it won't work with standard 3.5 inch drives.
While QNAP is very good, I often build NAS boxes using inexpensive parts plus a good RAID adapter -- usually an Adaptec 6805. You need the minimum of processing power and can run them headless from a network computer, although it is quicker to attach a monitor and set them up with the onboard video. 4Gb of ram is probably overkill, but it's so cheap why not? The case I use depends on what I have laying around, or if for a big array I'll get a cheap full tower with lots of bays or modify it. Whatever RAID card you go with -- get the kit that includes the 2 SFF-8087 mini-SAS to 4 SATA cables. They cost a lot more separately.