I think it really depends on what you want to do with your NAS. At least now you have figured out the hardware side of the storage solution, you need to also consider the software and workflow. The NAS features will depend largely on the OS and software you decide to run on the box.

If MS solution then there's no worry bout HW compatibility. You will have the advantage of testing out ReFS. Its supposedly an improvement on NTFS with build-in redundancies. Just note that Windowz is more resource hungry compared to other OS executing the same tasks plus you will need to defrag the HDDs every now and then. If you are proficient in Linux, you can do quite a fair bit with this HW spec. If ZFS is not your cup of tea, why not try Ubuntu/Mint with ext3/4? Runs more efficiently compared to Windowz.

For storage engineers, there's the consideration for capacity, performance, durability, reliability, power consumption and cost. You will have to find a balance between capacity and performance - the bigger the HDD, the slower the speed. And of course generally speaking, it will cost more to build redundancies and durability into the solution.

HW RAID card can help improve RAID performance however do get a UPS to protect against the write hole problem. In the worst case for the write hole problem, you can lose the entire RAID group in the event of a catastropic power failure and if the server happened to be writting to HDDs at that time. A UPS won't cost much and is a sure protection against this. Needless to say that if the RAID card fails, you will need to replace with the same model to access your data. If you don't want to mess around too much with RAID and just want minimal problems with max performance and still offers some reliability, then just stick with RAID 1.

Are you planning to use the server for anything else other than just pure storage? Coz IMO if just storage your CPU is overspec'ed. I checked the G2020 is a 55W processor, plus all the other components your power usage is probably hitting 75-100W. Not every cheap to run if you are leaving it on 24/7.
You do not need 1GB RAM per TB of HDD for ZFS. THat is only required if you are running de-dup
I also strongly recommend that if possible try to get ECC compliant RAM and MB

BTW what is the flashdrive for? If you are thinking of installing ur OS there my suggestion is to forget about it and get a HDD instead. You can get those 2.5" ones for cheap.