I'm a bit skeptical about opting for a fanless NAS because
(i) There are at least dozens of single-fan NAS products which are near to silence, albeit not dead silent;
(ii) The overall noise generated by 3.5 inch HDDs, quite similar to airborne engine sound in my opionion, will dominate the rest of the noise sources.
At the end of the day, most NAS products are equipped with CPUs barely suitable in terms of performance for a few selected basic NAS functionalities, which do not necessitate gigantic sophisticated cooling systems. Most of the time, the main source of noise is 3.5 inch HDDs. Also, perhaps without even a single exception, all 3.5 inch HDDs are quite noisy. I'm frequently distracted by the very noticeable noise coming from my WD Red HDDs (2x WD30EFRX) when I'm sitting 5 meters away from them, which are by the way supposed to be very quiet according to untrustworthy benchmark sites. To put another way, a fanless NAS does not really diminish the overall noise due to the presence of 3.5 inch HDDs.
I would buy Synology NAS products, e.g., DS213j, DS214j, DS212j, or even DS214SE, which are considerably cheaper than the aforementioned option, not to mention excellent graphical user interface, manageability, and near-to-silence operations. You can replace the fan of Synology DS21xj series with Noctua NF-B9 whenever you feel like to experiment it. As far as my experience goes, there is no need for replacement, though.
Main: SilverStone TJ08-E + Noctua NF-S12A PWM, Asrock B85M Pro4, i5-4430 + NH-U12S, DDR3 16GB@1600MHz, MSI GTX660OC + Alpenföhn Peter + 2x NF-S12A PWM, Intel SSD 330 180GB + 335 240GB, Corsair RM650
HTPC: Lian Li PC-Q11A + Noctua NF-A14 PWM, Asrock B75M-DGS, i3-2100 + Freezer 11 LP, DDR3 8GB@1333MHz, Zotac GTX650 + Accelero Twin Turbo II, Kingston SSD V300 60GB, Seasonic SS-400FL2