OP probably refers to Lenovo ThinkCentre mini PCs; they're well made, quite expandable and cheap, but if you can't find any, check out used Chromeboxes. They can be unlocked and reflashed with Coreboot in a single step, which makes them more secure aside allowing the native install of your OS of choice. Hardware quality is usually very good.
Chromeboxes aside, I've installed various Linux distros on different MiniPCs and the outcome has always been great; the only caveat is to check if your particular model does like to be used without a monitor. In some cases they didn't and refused to boot, but I solved by connecting on the HDMI port one of those cheap "virtual monitor", "dummy monitor", etc. dongles sold to perform this exact function.
From the above site I landed on the Dell WYZE 7020 / Zx0Q... Some models of which have 4 cores (also accidentally bought a dual core version), supports 16GB DDR3, are passively cooled without a fan, and cost me about $50 on eBay (add $20 for a 16GB RAM kit to bring the total to $70)
It's about as fast as a Raspberry Pi 4, and has been great for letting me spin up a docker container to play around with a database or a message broker or what have you.
I limited my search to only fanless designs though; if you are fine with fans then I'm sure there are more performant options out there.
Sibling comment has good suggestions. I personally like Dell Optiplex models, e.g. 9020. Looks like right now the best you can do is about 40-45 shipped for a complete one, but I've snagged about six in the last year or two for 20-35. They come and go. Beware of sellers posting the same thing for 100+; they're worth max 50.
