From the description of your work, for me I would go old skool recycler, build a pc out of parts, fill it with any old big disks, install Unraid or TruNAS, connect the machines with a pair of cheap dual port 10G network cards in link aggregation mode and of course, hope you use a UPS to protect everything.
If you think what you can get on ebay, you can make one hell of a storage unit ..
Big cheap case, with lots of HDD bays
Low cost quad core AMD APU, motherboard and 16GB ram, small SDD for boot drive off onboard SATA ports
Quality PSU based on power usage, but not a stupid wattage
and my favourites, DELL IT mode SAS controller card but use it in SATA mode and pair of x540 10G NICS, off ebay.
You could get all this for what, £150 of Ebay and of course the cost of cheap big SATA disks.