I did the opposite and used it as an excuse to upgrade my main PC, with the parts that got replaced being inherited by the new server.

Perhaps an unwise move due to it not being optimised for power savings, and looking at your particular use case it wouldn’t be a smart move.

Depending in where you want to have this NAS, one of the more important factors to consider is how quiet you can make it. If you only have a few HDDs they’re not too loud, but ssds are silent. It can also be worth getting some good fans and making sure you can mount them in a way that doesn’t cause unnecessary vibration to have it be real quiet.