If I use Charizard's "Roaring Resolve" Ability with only 10hp left, does he get KO'd when I place the 2 damage counters before or after I search & attach 2 {R} Energy cards?

You must finish all the text of the Ability before checking for knockouts, so yes you would search your deck and attach {R} Energy cards before KO'ing Charizard.

Relevant cards

SM9_14