When you ask a cordless drill to replace a gas powerhead its sort of a lot to ask.
The more inch lbs of torque the better as far as cordless drill.
How much is enough? I'd be looking for 700 in lbs or more and then 4 amp batteries.
If it can spin a 8" SM Lazer it'll have no problem with a 8" Nils. The coefficient of the Nils means less torque required to do the same task as a SM Lazer. As far as drill and battery means more hole drilled with Nils. While the cordless drill and auger seems slick and lightweight, longevity of the drill is IMO going to be short. Yes I know the Milwaukee Brushless Fuel is a beast of a drill and on a 6" auger I'd be confident I wasn't over doing it. Step up to and 8" and 3' of ice, I'm telling you that drill is going to get a work out.
Why am I talking about 3'+ ice, because when you drill it it leaves a lasting impression and thats what we get here in Northern MN.
If your real hot on going electric the Ice Gator Annihilator and Nils is something special.
It'll handle the 8' bit and 3' of ice and ask for more. Plus if you go that route you'll use the standard Red Nils blade. The IG will have plenty of torque so your speed of cut will be maxed out. You'll also hands down have the faster electric auger on this Earth.
It'll make the ION look like the training auger that it is. Sorry ION guys
Then again if you get 12" or less of ice then a drill or by hand is the way to go.