Raspberry pi is an option, but what I prefer is building a desktop pc, because its way faster and upgradable. If you don't need much power, you can get an athlon 3000G, a B450/A520 motherboard, and 8GB of RAM for under €200. Add a cheap ssd, 400W PSU and Ubuntu server (LTS) and your server is done (you dont even need a case, just use the motherboard box). O ...
If you are buying new machines, why not going for AMD CPUs. More cores per dollar ratio.
If you insist on going with Intel's platform, I'll highly advice using Xeon W 1x70 series (out in 2021) therefore faster RAM support.
PS: For proper DB performance, indexes etc.. Go for proper amount of RAM depending on your database's size.
I haven't used it myself but one I've got my eye on with a view to maybe getting a couple of them in the not too distant future is the Kingdel NC860 Mini Desktop Computer (https://www.amazon.co.uk/dp/B087JCPDZB/ref=twister_B087M15RVB?_encoding=UTF8&psc=1).
They have a range of models but a lot of them are dual-core, so watch out for that.
As said, I ...