Used Electronics on Ebay
I have bought a lot of used components from eBay and maybe I’ve been lucky but a vast majorly have worked without issues. Typically when I do get a part that is DOA or has some major issue I have been able to work out a return or replacement with the seller. Ebay is buyer beware so pay attention to things like the sellers feedback and of course unlike buying new you won’t get no-questions-asked returns or warranties.
Chinese Sellers
I have bought some stuff from Chinese sellers and have not personally had any issues with broken or fake parts. I do typically try to buy from sellers in North America and will often pay a bit of a premium but that has more to do with shipping time. The main downside of purchasing from China in my opinion is just that it takes a long time to receive your purchase, typically 2 - 4 weeks as apposed to a week or less for domestic sellers.
RAM
I wouldn’t sweat the brand of RAM. Premium ram with high speeds and low latency are typically targeted at gaming PCs and even there I’m a little skeptical it really makes a difference in most cases. For your Home server / NAS you won’t see a performance difference and the main decision you need to make is whether or not you want ECC. Pretty much any DDR4 Ram these days will have no issues running 3200Mhz with loose timings so cheap Ram from any name brand manufacture should work just fine.
HBA
When I mentioned that you might not need an HBA I was assuming you had not bought drives yet and was suggesting that yo get SATA drives and just use the onboard SATA ports on your motherboard. If you have SAS drives you will need an HBA.
I would recommend getting a used LSI HBA. There are a bunch of them but most any of them should work. Probably an LSI 92XX-8i or 93XX-8i I would expect to pay between $10 and $50 USD depending on what you get. You will need SAS data cables and they will work with your SATA power cables from your power supply. Even if your drives are technically 12G SAS you can still use an older 6G SAS HBA and get the same performance. You won’t hit the 6G limit with spinning rust, 12G only really matters with newer faster more expensive SSD SAS drives.
If the HBA is a Raid card you will want to flash it to IT mode before attaching drives. I would recommend against using Hardware Raid in general.
Hard Drives
The price for those 16TB is much more reasonable in CAD than the USD I had in mind. While I do hold that used electronics are fine, most don’t wear out under normal use; Hard Drives are the exception. They do have moving parts and they do wear out over time. I still typically go for used drives as, at least where I am, they are so much cheaper that I can buy a couple extra drives in case one fails and still come out ahead on cost.
Enterprise vs Consumer
New out of the box enterprise drives should be more reliable and have a longer service life, but the flip side when buying used is that the enterprise drives have been used in an enterprise environment and likely have a lot of miles on them. Enterprise drives are very expensive new but tend to be very cheap used. I suppose my recommendation would be to buy new or lightly used consumer drives with warranties or cheap used enterprise drives.
I typically go for used enterprise drives and some of them do have some pretty high numbers racked up in the SMART info, but I figure that my usage is very light compared to enterprise use cases so hopefully the drive will continue to work for a while yet (knocking on wood). Your mileage may vary. What you choose is going to be a bit of a calculus based on your budget, your capacity needs, how much you trust the used drive, and how much you value a warranty.
RTG
A good resource for used enterprise drives, HBAs, and other used enterprise gear is Rhino Technology Group on Ebay. He is an active member on the Serverbuilds forums ( easyrhino) and will post deals here and will often extend buy it now offers to other forum members. He is a legit seller and lots of folks on here have bought from him. The international shipping prices on Ebay may be pretty high by default but if you reach out to him directly on Ebay he may be able to reduce the shipping costs some for you.
Capacity
I’m not necessarily saying to get lower capacity drives, just pointing out that 2 drives is a worst case scenario as far as how much capacity goes to parity and that you might technically get more capacity per dollar using a larger number of smaller drives.
That said one of the biggest advantages of Unraid / JBOD over a Raid based system like TrueNAS / ZFS is that it is very easy to add additional and presumably over time you will add more drives to your array. Once you start to fill up your NAS with drives the higher the density the better as that will determine your max capacity when full. The right size drive is something you will have to determine for yourself based on your budget, current capacity needs, and your future expansion plans. If you have less than say 10TB of data now, but think you might add 1 or 2 drives over the next few years then 16TB or even larger might be best.
Unraid and Cache Pools
A good resource for learning about Unraid is the youtube creator Space Invader One. He has a bunch of detailed guides on the different aspects of Unraid and I think he does a good job of explaining both how to set things up and also why. Here are a couple of video he did on working with cache pools:
https://www.youtube.com/watch?v=ij8AOEF1pTU (This one is a little out of date with the current version of Unraid but the concepts still apply)
https://www.youtube.com/watch?v=vXF8au5o9Tw (This is a more advanced use of ZFS and cache drives)
Good luck with your build.