I just created a NAS from 2nd hand parts. However, I was lucky enough not to need to save any data from a previous NAS, since I didn’t own a previous NAS… 
That said, I went another route: HP Microserver G8 with 4x WD Re 4, 4TB disks. Granted, that is a lot more expensive than your solution, but I didn’t spend THAT much:
- HP Microserver G8 (2nd hand, with 8GB memory): EUR170,-
- Intel Xeon E3 1260L (2nd hand): EUR30,-
- 4x WD 4 Re 4TB (2nd hand, about 35k use hours each = about 4years): EUR250,-
- 1x 120GB SSD (new) for ZIL: EUR30.-
- 2x thumbdrive 64GB (as raid1 boot/system disks)
I installed TrueNAS 12.0-U2.1 on the thumbdrives and created a RaidZ pool from the 4 WD disks.
Plan is to have the NAS as an NFS target for 1 or more Proxmox compute nodes. Now putting money aside to buy the first compute node. Hopefully somewhere before summer starts.
I only have bumped into 1 minor thing: since I boot from USB thumbdrive and the HP ms G8 refuses to boot from the USB3 ports, It takes a bit of time to boot. It also means that extra plugins take like forever to install and start. So I have to refrain from plugins to make the system stay usable. That’s no biggy for me since I plan to host any services on a VM or container on Proxmox later.