I would recommend a VPS with 2 cores and 4GB of RAM. I tried running Plebbit on less, but it crashed regularly. However, I set up a systemd service that automatically restarts the Plebbit daemon if it crashes, so I don't have to worry about that anymore. The only annoying thing is that plebbit doesn't start your subs automatically, so I added a shell script that starts the sub after 30 seconds. Since then I had no issues.