Thanks all, for your words.
I'm curently planning the E-assist.
Bought a 36V/20Ah LiFePo4 a few weeks ago and tested it on a friends bike, in this case a rather heavy pedal quad with a 350watt mid drive motor.
It was real fun to pedal uphill and feel the airstream around my ears
Once i was used to the assistance, switching the motor off feeled like driving with brakes.
I pedalled 10km with around 250metres/775ft of elevation for testing.
Judging on the voltage, the battery was still 75-80% full after the test.
Now I'm searching the best solution to include an e-motor into the left drivetrain of the Roadster.