If the only issue is running out of disk space, why not just buy a larger hard drive to replace the 500GB with?
Also, ECC is always helpful, whether you run truenas or not, but in no way is it a requirement for truenas more than for any other OS. If you want to read more about it, click here for Jim Slater’s blog (a fantastic ZFS resource).
What about just getting used ATX/mATX MBO for your socket? And any mid/high tower pc case would suit your needs. Not sure can you use PSU from dell, but even if you have to buy one its still cheaper than full pc build
I don’t think it would be much cheaper. Because it’s a micro PC, with an external power supply, I’d rather keep that one as is and then just buy a new machine.
For example, I saw a Dell Precision T3620 Intel Xeon E3-1245V5 with ECC memory for about 160€, and it looks like it may do the job more than adequately.