you can use Eternal Items to start with if you want to, but you can barely use any Crystals on them.
eventually you will use Legend Items for your Builds, and maybe have one or a few Eternal Items if they fit in your Builds.
a General Farm Build just needs enough Luck & Gold Find to reach the Cap, and the rest of the Build is for killing monsters and staying alive. having better Item Drop Rate increases how much loot is dropped.
if you want your Farm Build to focus on something, like finding Eternal Items, get items with the Set that improves your chances of getting what you want.
the rest is up to you based on your play style and which weapons and skills you like to use.