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|NF-A14|NF-S12A; Asus H110I-PLUS D3; i5-6600K|NH-U12S; 16G [email protected]; Palit GTX750Ti|Accelero Twin Turbo II; Intel 330 180G; Crucial BX100 500G; Corsair RM650; Dell U2713HM; BlasterAXX SBX10; Audio-Technica ATH-MSR7
HTPC/Gaming: Raijintek Styx Red Windowed|3x NF-S12A; Asus H97M-E; i5-4430|NH-L12; 16G [email protected]; Asus GTX970 Strix OC; Intel 335 240G; SilverStone ST30SF