It must be a pedelec (pedal electric cycle), that is legal in Germany (StVO comply) and considered as , no license plate and insurance required.
Preferable budget is 300-700€. More than that is OK, but do not exceed 2.000€ (after discount).
Small and must be foldable.
Around 10-30 kg.
Around 16-20 inches. Avoid fat tire (more than 5 inch wide) as possible.
Preferable mid-motor, but rear hub (geared) is OK.
Preferable torque sensor, but cadence sensor is also fine.
Bigger capacity is better. Higher voltage is also fine. No preference. Preferable internal battery.
Preferable front and back suspension. Or at least it has one in front.