Update: see Quickstart Error: npm does not support Node.js v10.19.0 - #5 by peterparker
Regarding npm issue, generally speaking it is a bad practice to use node/npm with sudo, therefore, instead of fixing the sudo npm command I would advice to try to run the commands with your user npm and then fix the potential errors there.
dfx itself might need sudo though, this is how I fixed it on my osx (Permissions on OSX - #2 by peterparker)
P.S.: you can also try to generally upgrade node on your machine, current LTS version is v14 but yep, again, better to run it with your user.