I would say with a broadside shot at 20 yards and under, 30-35 would be an absolute minimum. Though at this level of KE I'd be more worried about momentum (i.e. usinga heavier arrow) and also would suggest a stout cut to the tip broadhead.

I personally don't care for anything under 65 pounds of KE for shooting mechs ('cos I like those with larger cutting diameter), and prefer to have at least 70 pounds of KE.

The more the merrier as long as you are not giving up accuracy/killing yourself to get the bow back.