Melfox

ZFS stands for Zettabyte File System. Even with the highest capacity drives you can't fill a server with that much storage but besides that supposedly you could create a share up-to 1 zettabyte in size. So there's no bottleneck but UnRAID should be capable of huge capacities as well.

So from that product page it is complete crap to find the info. You have to click or scroll down to support. Click on "Product support". Click or scroll down to "Compatibility" Then Click on " Compatibility of Desktop 8th Generation Intel® Core™ Processors "
 
Then you find out that it's indeed not compatible with any intel 100 or 200 series chipsets.

8th gen chips will only be compatible with 300 series chipsets (H310, H370, B350, Z370), although only the z370 exists at the moment. 
 
Well, I wouldn't recommend exposing IPMI to the internet -- I would just setup a VPN on the NAS or on the router if it's supported. Then you can just VPN into your home network to remotely access the server. 
 
Well, as you mentioned, you can just create a second array when you want to add more drives. 

FYI the i5-8400 is not compatible with the H270 chipset. The 8th generation chips currently are only supported by the Z370 chipset, although there should be some cheaper options coming soon.

For a custom built NAS IPMI can be a very handy tool. There's supermicro which is a very well known manufacturer of enterprise grade server motherboards but personally I've gone with ASRock Rack. Their IPMI is easy to set up, understand, and remotely manage the server (Java required). I've also had good luck with their RMA service. I had a board fail 1.5 years into use with a 2 year warranty. They replaced it with no hassle.
 
I'm not sure about UnRAID but with FreeNAS you'll want ECC (Error Correcting) memory and a Intel Xeon CPU with ECC support not a desktop CPU like you listed. UnRAID I think would benefit from ECC and a proper server CPU as well.
 
I don't know many of UnRAID's features but something I'd recommend against is utilizing it just so you can mix drives of different sizes. That's basically JBOD (Just a Bunch of Disks) a super outdated un-recommended form of RAID. A proper array should use drives of similar spec.
 
Again I don't know about UnRAID but FreeNAS has the option to create multiple shares which can be assigned to independent RAIDs or broken down from a single large array (similar to partitioning). These can be assigned owners or groups and even password protected for the local network. You can enable SSH (Secure Shell) to allow you to remote into the server and have access to each share that you have permission to access.

It's something built into the motherboard -- anything at a software level wouldn't be able to turn the computer on/enter and control the bios/etc... For example, on Supermicro boards it's called IPMI. It's not a necessity, but I wouldn't personally touch a board for my NAS that doesn't have some kind of remote management hardware.

The stock cooler would be fine, you likely wouldn't benefit from a cache drive, and the stock fans should be fine.
 
although I'd go with an R5 1600 instead. I'd also personally look at a server motherboard that has some kind of remote management just for the sake of convenience (e.g. I can connect to my server while it's powered off and turn it on/interact with the bios/the OS once it boots just by the IP address). These server motherboards are also likely to have onboard graphics too. I'm not sure if there are any Ryzen boards with remote management though. 
 
You can buy molex to sata adapter chains, so the PSU won't be the limiting factor. Physical space will be what limits you. The case you chose is limited to seven 3.5" drives and three to five more if you get a 5.25" to 3.5" adapter.