Depends on really what do you want to do. For just data storage, NAS, pretty much anything goes. If you want to run VMs, Docker etc. you want something newer. Personally I use Optiplex Micro 3080 with i5-10500T. That was 270€ refurbished.
>Do any of these support ECC ram
No, as far as the Dell, Lenovo, HP mini PCs goes.
>And would you say mirrored ZFS is good enough for home use?
Yes but reading your use case you probably don't want a mini PC. You can only have 1x 2.5" SSD and 2x NVMe SSD. A single 8TB NVMe SSD is currently 1000€ and you would need two. Unless you want something smaller of course.
Depends on really what do you want to do. For just data storage, NAS, pretty much anything goes. If you want to run VMs, Docker etc. you want something newer. Personally I use Optiplex Micro 3080 with i5-10500T. That was 270€ refurbished.
>Do any of these support ECC ram
No, as far as the Dell, Lenovo, HP mini PCs goes.
>And would you say mirrored ZFS is good enough for home use?
Yes but reading your use case you probably don't want a mini PC. You can only have 1x 2.5" SSD and 2x NVMe SSD. A single 8TB NVMe SSD is currently 1000€ and you would need two. Unless you want something smaller of course.