Hello, and welcome to the forum-
I'm not sure how to solve your eth0 situation, but the "command not found" issue goes like this.
Many commands are in directories (those "sbin" ones) that only root has a PATH configured to.
When you use "su" to change to root, you become root but you do not inherit roots envornmnet
variables, so you do not gain roots full path access. To change to full root configuration, use "su -"
(that's su followed by a space and then a hyphen).
Otherwise, you can just specify the full path to the command: /sbin/lspci
You can use the 'which' commnd to locate where a command resides: which lspci