Want to build home nas

Used to building PCs but new to servers. I want to buy some used parts, if a better deal than new, that would have moderate performance for a nas but really care about ecc memory and at least 6 drives. Thinking using ubuntu server with zfs or freenas. I’m just perplexed with figuring out good used equipment that is affordable. I would like a desktop size or preferably miniITX but I know that might be too difficult.

Thank you for your time.