With anything we are all going to have differing opinions on each of your questions. Personally I think the first thing you need to do is decide on a budget and your personal requirements (speed, range, how much assist do you really need, are you riding hills or flats and are you capable of a DIY project or better off buying a complete bike). You also need to decide between a hub motor or a chain drive setup. A person that requires very little assist and rides slowly on flat paved roads will get very good range but likely not close to the quoted range of the manufacture. A good rule of thubm is about 50% of the rated range but normally the speed quoted is true. However, that can depend on your weight too. As a rule the higher voltage setup will be more eff. as will brushless motors. However, if you are on a tight budget I would go with a brushless motor setup and put my money in a lifepo4 battery pack. The battery is the most expensive part of the ebike. Be careful of low cost kit a lot of people have reported batteries sold with these kits were undersized and failed quickly.

I have 4 bikes for my wife and I . 2 with Cyclone motor kits mounted on Wallie world bikes and 2 Ezips complete bikes. I can flip a coin which bike I like better. There are times I prefer to ride each of them. I like the Cyclone bike because it's lighter and I prefer the location of the battery packs. But I like the looks of the Ezip better and the fact that I can get cheaper parts when I need to do repairs. Maybe someday I will melt them together and have the perfect bike. Note..all my bikes are chain drive and 3 of them drive through the bikes gearing setup. This allows for better hill climbing and better max speeds. The value of a hub motor is they are generally quieter.

I know this didn't answer all your question but maybe a few.

Bob