From my experience the $500 electric bikes won't be able to take commuting any distance. A conversion kit with an upgrade from SLA batteries and your donor bike is a good place to start. You might want front and or seat post suspension as well to smooth the ride. A couple of good panniers on the back will solve your 'carrying stuff' problems.

I've been commuting 42 kms a day on an e-bike for 2 years and I find the Wilderness Energy and Cystalyte hubs very reliable.