As everyone have said, it depends on what you want to have in your server. I started with an old lenovo I bought in mercado livre for 200 BRL. It was a DDR2 PC with 4Gb ram. I bought an ssd and installed Debian. Used for years. After that I tried to build a DDR3 PC. Made it with 800 BRL and it’s decent to run my docker containers like an arr stack, nextcloud, VPN, reverse proxy and vaultwarden.