thiocyclist - what to look for in a conversion bike depends on how you plan to use the bike.

If you plan to regularly ride at faster speeds - say 30 mph your needs will be different than those who are looking more for a cycling experience, with assist. In general, faster = more powerful, less range unless you add more battery = more weight and higher speed = rougher ride = shocks are a good idea and if you want to stop a heavy fast bike you need more powerful brakes. Many e-bikes weigh well over 50 lbs.

As for brakes, my wife and i ride a tandem bicycle and live in a hilly part of the country. The total weight of riders, bikes and gear is about 350 lbs and we have a nice set of regular caliper brakes that work extremely well. But if you and the bike are heavy and you will be riding 30 mph and you plan to commute rain or shine, disc brakes are a good idea. Disc brakes may be "better" in many respects but depending on your needs, rim brakes can be more than adequate.

I am in the process of building an e-bike. I am a cyclist who is looking to make things easier for myself. I am purchasing a steel frame and fork, and will have many options if I want to experiment and try different things. I like the ride of a steel bicycle and will be able to mount a hub motor front or rear w/o torque arms or go with a mid-drive. The frame has horizontal drop outs so I can use an internally geared hub, go single speed or use a conventional derailleur drive train. Although I can choose a fork that will mount a disc brake, I am going with the rim brake option. I am making this choice because i am building the bike for a top speed of 20 mph and total weight of the bike with motor and battery will be 36 lbs.

Another thing to consider is how much room you have in the triangle for a battery. Some frames leave very little room. Many people are happy with simply mounting the battery on a rack over the back wheel but I really want to have the battery located low and centrally - a bottle mount battery or frame pack if more range is needed.