@tablespoon
If you're wiling to drop money on a mac to turn it into a NAS let's talk budget here.,
If you get a used PC and throw a 3.5" SATA drive into it and install Linux / Samba on the boot drive you basically have a NAS. Most NAS devices start out at $200 and you can pick up a PC for less and just tinker a tad to make it into a NAS w/ better performance and flexibility to expand later or up the NIC speed if you need faster network speed for a lot cheaper than most prebuilt options.
I run a "NAS" inside my "server" w/ 5 drives in Raid 10 with 1 being a hot spare in the array. I've worked with plenty of NAS devices to know the limitations and is why I recommend building one vs buying one. They're fine until you need more options / expandability / faster network access. The other issue is some of them have really bad security issues in their OS that come up from time to time and cause people to lose data from remote wipes.
The drive alone is going to be ~$100 depending on which size you want to go with
The PC can be as cheap as you want to gamble on but, a new SFF PC run s about $150 with somewhat decent parts inside. Upgrades are easy to do and relatively cheap if you want to boost the bandwidth.
The issue is usually the bandwidth bottleneck of a 1gbps port only gets you about 125MB/s whereas the drives these days usually hit 200-225MB/s. In my Raid setup I actually push ~450MB/s which means my network is setup for 5gbps to make sure there's not a choke point for the data being moved. I tend to do most stuff over WIFI though and can get 1.5gbps through my setup as it stands today but, when I want to copy more intensive data I just hook up an Ethernet cable directly to the server and get max speeds.
Moral of the story is you can drop ~$500 on a NAS + drive or you can DIY something better for cheaper or the same price.
PC - $150
NIC 5GE - $100 or less
Drive - 18TB could be $300
Another perk of this setup is you can run things like Plex from the PC with less buffering issues than a NAS if the file isn't natively playable. The CPU will handle the transcoding better than any cheap NAS up to the point of breaking $1000 for the ones that have decent CPU's in them // I3/5/7.