I bought a pretty basic no-frills bike (https://www.feltbicycles.com/USA/2016/Bikes/lifestyle/Urban/...) and used the same conversion kit that the author of this post uses (Bafang BBS02 from LunaCycle - less than $100 for a local bike shop to install). I've been riding my e-bike to work for several months and the caliper brakes that came with the bike have held up fine. They have enough braking power that it feels safe, and don't seem to be wearing too badly. So I don't think you need disc brakes at all.
The only problem I've had with the underlying bike is one incident where I slammed on the throttle from a full stop and the motor wrenched the whole back axle out of its alignment and jammed the wheel at an angle to the body of the bike. I guess the nuts securing the axle had worked themselves loose. Luckily I was close to work and was able to walk my bike in, and bring some tools the next day to loosen and re-tighten the nuts.
Overall I think it's been great. I wouldn't go back and buy a more expensive bike if I could. The nearly car-like prices of "good" bikes are too much.
Good steel frames are not necessarily significantly heavier than aluminum frames but they ride much much better (have more flex). They also can be repaired if required while aluminum is probably toast after anything that damages the frame. In anything that is not a race or cargo bike I’d always prefer a steel frame.
Steel frames are only easy to repair if they're lugged, i.e. the tubes are brazed into lug sockets. This was once a common method of hand building frames, but it's now rare and expensive. See https://en.wikipedia.org/wiki/Lugged_steel_frame_constructio...
Compared to aluminum frames, all steel frames are more easily repaired. They can be welded without special equipment, put back in shape when slightly bent etc. They also fail less catastrophic - if an aluminum frame or fork receives even a slight bend in a dooring accident it needs replacing since it may just break clean though at the next inconvenient occasion (like hitting a slight bump downhill with high speed)
Cargo bikes require significantly thicker frames to sustain the load, so the weight difference is much more pronounced. They’re also quite heavy to begin with, so shaving an few kilos off is most often worth it. There are some bikes that actually have mixed steel/aluminum frames, Douce bikes for example use steel for the back part of the frame where the rider sits and aluminum for the front cargo section.
It works great. I was specifically looking for a bike without gears since I read that many people with e-bike conversions just leave it on one gear all of the time anyway and I didn't want the apparently-unnecessary additional complexity and failure modes of a derailleur.
Besides, the pedal assist power is adjustable so if I hit a hill I can just press the + button on the controller on the handlebar and I get more power. That functions like shifting into an easier gear, without having to pedal like mad to get anywhere.
The only problem I've had with the underlying bike is one incident where I slammed on the throttle from a full stop and the motor wrenched the whole back axle out of its alignment and jammed the wheel at an angle to the body of the bike. I guess the nuts securing the axle had worked themselves loose. Luckily I was close to work and was able to walk my bike in, and bring some tools the next day to loosen and re-tighten the nuts.
Overall I think it's been great. I wouldn't go back and buy a more expensive bike if I could. The nearly car-like prices of "good" bikes are too much.