Spell damage will work for the original melee strike, but it's rare that your original hit is your main damage. Not worth investing into IMO. And spell factor does nothing.
For flat damage, you'll need to get lots of -enemy res of whatever element, and/or global elemental damage (i.e. firedance or acid fiends). The damage also scales with the WDM modifier of your attack, if you have options there.
For physical damage converted, it's the same, except that anything you do to increase your physical damage increases your elemental damage as well.