Procure a nice late 80's to mid-90's steel MTB and equip with a BBS02 or BBSHD @ 48 or 52V. Easy conversion and (for my wife and me) durable bike. Alternatively, use a road bike if that suits you better. Procure a 42T Lekkie chainring for a good chainline. I prefer 68 mm bottom brackets since it's possible to use the "jam" not to secure the one on the motor.