Seeing as you are located in Switzerland, and shooting for a vehicle that goes up to 45 km/h speed, the Stromer has that one critical advantage: It can be registered ("gelbes Kontrollschild") to be legal on public roads, as a so-called "S-Pedelec".
If you shop around, for instance on https://www.stromvelo.ch/ebikes/Stromer (no affiliation, no recommendation etc), you can find road-legal, extremely well-supported, fully warranted Stromer ST1X at CHF 4200 = USD 4200 including tax (model year 2017).
I'd expect the Swiss police not to be totally stupid and recognize an "abnormally" fast electric bike reasonably well, in particular if it lacks the number plate. I suppose that they most often will take a blind eye to that, until there is that single unexpected incident, with insurance and whatever else involved.
For the record: I ride a very cheap hub motor DYI, which I very deliberately hold back. CHF 450 _in total_ (electric components) got me something into my fully MTB that can be ridden 40 km/h all-electric in the flats, at 18 kg. *And* still I am looking for an inexpensive used Stromer ST1X or Stromer ST2
Recommendation: Have a good at smide.ch and rent a Stromer for an hour or two (Basel, Zürich).