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Here's the part list: https://uk.pcpartpicker.com/list/fcxgk6

I shopped around and waited for sales/price drops and got it all for under £500 (not including drives). I don't the pricing if your're not in the UK, so it may be cheaper elsewhere to do another combination.

As for drives wait for good deals on WD Elements/My Book Desktop External Hard Drive and shuck them. Note, only do this for +8TB drives (these are CMR drives atm) as under that size the drives are SMR which you should avoid like the plague for RAID/ZFS as they have been reported to fail if you ever need to rebuild. So then you're looking at 4x£120 which is another ~£500ish investment.

If you want to save money get a preowned box like the N54L off eBay at ~£100, and spend the money on drives, then stick FreeNas on it and if you're only file-serving the box is good enough.

I think the custom box is a bit overkill but it's got so much room for upgrades, I plan on getting 8x10TB drives in there one day, plus you can run VMs on it as the i3 processor isn't too bad.