Is there something you would like to caution me on?
Since you were told to buy used SSDs the first thing I would do is checking the SMART data (how many 'power on hours' and if existent the amount of wear out -- buying used SSDs without knowing their health data is buying a pig in a poke. If they were not used that much they can work fine for decades... or simply stop working soon if already almost worn out).
Asides that with the use case you described I personally don't understand why one wants to spend hundreds of bucks on something that will neither perform faster nor more reliable than for example an ODROID-HC2 (and wasting a lot more power at the same time). But I would assume your project is more about 'building a PC' than 'NAS'?