What kind of life is required for practicing Metta?
"content &amp; easy to support" is Santussako ca subharo ca.

There's a fairly long description of that, with sutta references, in this definition of asantuṭṭhitā.

I think that "easy to support" is one of the things that the Buddha said that monks ought to be.

It's a formula, a phrase that's used in several suttas (I imagine it's in the Vinaya too).

The commentary says that the Karaniya Metta Sutta was addressed to a group of all/only monks.