I strongly suggest to register with The New Motion
before starting the trip. Registration is free. You only pay what you use.
Have your phone charged and the apps installed and configured.
Is your i3 capable of charging via CCS?
If so, I suggest this trip: http://www.goingelectric.de/stromtankstellen/routenplaner/1102606/
The first stop is allego which you can definitely pay with either of those apps.
The second stop is the tricky part. It's a rather slow Type 2 station - but you pretty much have no choice here because you drive through the worst possible area in Germany when it comes to charging stations. This station should be usable with either of those 2 apps as well. You'll need a charging cable (Type 2)
here. Make sure to get one when you buy the car!Correction
: The last costs money now too! You pay with cash at the register.
The longest leg is 108 km which should be no problem unless you get careless and start to race.
Feel free to message me and I will send you my telephone number. Call me if you get into trouble and I will try to help you.