Buy refurbished Mini PC with at least 16GB RAM and 1Gbit network card. Don’t bother with M.2, SSD is enough.
I have right now refurbished hp g4 ryzen 5 mini pc. There is 12 docker containers running there and couple applications in screen, I just add things. Don’t do ARM, as far as I remember running on ARM is compilation hell. Stick with x86. You want it to ease your life and not to make it harder.