Current planned use cases are Pi Hole, Bitwarden (password manager) server, probably general network storage and torrenting, whatever other "homelab" things I think of later (which is why the specs probably seem a bit overkill)
Lenovo ThinkCentre mini desktop, eBay refurbished, $184 before tax
8GB RAM, 500GB SSD
Intel Core i5 6th Gen. (they don't specify but I'm assuming it's a 6000 series, which was released in 2016)
Direct link to the listing: https://www.ebay.com/itm/293526962748
New HDD or used?
HDDs are a lot more sensitive to travel/shaking, I wouldn't buy one that isn't brand new from a legitimate store. Or if you're running a used one, then run two mirrored so you're not boned when it dies.
Used SSDs you can check the wear level after it comes to know how much you should trust it. HDDs are more of a toss-up.