Many hardware vendors supports FreeBSD, I don't think anyone running ZFS would advice you to use Linux but oh well... There's a recent presentation about how gandi.net uses FreeBSD and their infrastructure and why they choose that OS. BSDCan2016: FreeBSD based high density filers
Napp-it is also...
Marvell controller, preferably MLC instead of TLC memory. Proven and reliable combination.
Crucial MX200, Plextor SSDs also comes to mind if you're on a budget.
At least in .eu they have quite a bit more powerful hardware at the same price as Asustor. Keep in mind that if you're going to mix HDDs sizes you can pretty much forget about redundancy in a sane manner at least.
Well, I do know that the SiL3114 are plauged with data corruption even on smaller HDDs if you use more than one channel at a time (that's a very nice design bug) . The SiL3124s aren't much better, also... it's going to be really slow and PCI isn't going to help. I'm recommending PCIe because no...
It's hackable but it's not a good idea in the long run.... Asustor (better bang for the but than Synology and QNAP in most cases) supports NTFS out of the box from what Google tells but I would recommend you switching to a native filesystem. FreeNAS has the same "issue".
Slower than the Fitlets (they are slightly more expensive) however you'll be more than fine with a MIPS or ARM box which most likely will perform better. Not to forget, warranty is a concern if you order overseas.