"Downhill" mountain bikes is a term of art and, as an avid mountain biker, I'd guess not actually the type being used for conversions to commuter bikes.

You're much more likely to target a "trail" full suspension mountain bike or fixed tail bike. You'll have a lot more cost options and they're more suitable for a commuting purpose, they'd still have plenty of travel and braking compared to a standard commuter or utility E-bike. As a note, my mid-level trail bike came with a better set of brakes then my high end cargo hauler e-bike which weighed about 10 times as much.