Answering my own question about an electric learner car, the Mitsubishi i-MiEV/Peugeot iOn/Citroën C-Zero looks very promising:
- up to 14 years old now
- super compact
- high mileage
- parts relatively easily available, shared with gasoline relatives
- stable electronics, mostly shared with gasoline relatives
- simple electrical system with little or no encryption/DRM/..
- DIY battery rebuilds known and fairly well-documented