As someone running a pihole off an old radio 2B, your MacBook will be more than sufficient to run what is needed. My only advice would be to get an Ethernet adapter if that model doesn’t have one. Losing valid dns queries due to wifi packet loss would be annoying. Beyond that, just google a guide and go, it’s super straightforward to set up and manage.

That’d be fine. Laptop cooling fan might die from being on all the time as well as mechanical drive issues at that age but it’s solid hardware otherwise. Pihole is not overly intensive. Ideally make sure the pihole machine is on a wired network connection inside your LAN, because wifi routing latency will be bad otherwise. So that may necessitate a thunderbolt ethernet adapter, but I’ve bodged together much worse before lol.