The vehicle is going to need some smarts put back in it once the ECU is removed, to manage things like the dash instruments, and act as something of a state machine for different modes – charging, driving, parked etc. I’ve bought a powerful little microcontroller for this – a Teensy 3.2. But my initial experiments will be with a chip I know better: the ESP866. I have tons of these around the house as sensors and remote controls for sockets. They’re cheap but capable and have built-in WiFi, which makes it easier to get data off them.
[Note: this is a holding page. I destroyed that first Teensy. And the second one. And the closest thing I have to a VCU at the moment is a mess of diodes and relays. But it works.]