The level of system integration in a modern car is huge. So many systems are interdependent, unless you have detailed knowledge of the electrical architecture then you have no idea if any of the systems will work when you take one part of the system away. It’s not just the dash and abs/esp, it’s also the power steering, body control module, alarm systems, the list goes on. Some ECUs also act as gateways for CAN systems, so take one of those away and other parts of the car will start to play up in a way that you might not expect. You also have network management functions which are very sensitive to the power up sequence and can prevent the car from shutting down correctly if not respected. Ok a mk1 leaf might not be at the cutting edge of modern day electrical systems, but cars going back 10 years – such as the leaf – will still be complex.
Messing about with modern cars isn’t like restoring an old Porsche, Land Rover or camper van. It’s not a case of just a bunch of wires providing 12v and a ground, it’s way beyond that. The point I’m making is that the OP could spend a load of time and effort getting the mechanical systems (roughly) together, but then he might find he hits a brick wall because of a software integration issue that he has no chance of fixing. On the other hand he might be very lucky and not have any issues, but i would be surprised if this is the case. If he wants a fun project where he changes the driveline, then an old land rover / Porsche would be a much better bet, and be a nicer car than a leaf at the end of it all too.