Y'all might want to check out a couple of US sites. There is a ton of information available.
http://www.electricbike.comThis site is perfect if you want to do a DIY build of an existing bike you treasure (or just have). Lots of info on components types and quality, many examples of builds from really crude to rather sublime.
Lunacycle is a US bike builder who buys bikes wholesale from the manufacturers and then "builds them" for you - often at prices significantly cheaper than if you buy an ebike from an ebike manufacturer, and even cheaper than if you bought the bike and components separately. Probably wouldn't make sense to ship from them to Europe, but again - a lot of bike info there.
https://lunacycle.com/KTM recently announce that they are "all in" on ebikes, and that the distinction between bicycles, ebikes, and electric motorcycles is going to blur very quickly. They have had an excellent electric dirt bike in testing for a few years now.
Apparently in Europe the proliferation of bikes and scooters has caused regulators to have a significant advance or lead in developing rules of the road regarding ebikes. There are several classes developed based around pedal only, pedal assist, pedal assist with throttle, or full throttle bikes.
In the US it's mostly still the wild west - with only a few jurisdictions even knowing this stuff exists. Generally if it still looks like a bicycle and you don't act like a fool you can ride most places. This will change over the next several years as bad actors cause problems that get noticed by authorities.
I've been lusting after one for a year or so - but the one that I want costs so much I really have to justify that I'll use it enough for it to make sense to buy.
Best of luck