For parts it looks like about $20k, the labor is a bit of a question. I'm fortunate enough to have friends who are retired professional fabricators and race car engineers, who have the facilities, tools, and skills to do that part of the job well and for beer money. The electrical is pretty straightforward, as all this has been done many times before and is well documented. The big questions are around the brakes and HVAC. Brakes are currently vacuum assisted and will need some figuring out. Heat and AC are an open question.
With just a single power unit it will only be a 15-20% increase over current power levels, and the total weight and distribution won't be too far from what it is now. Of course, fast car crashes hurt more than slow ones, but that is a different discussion.
Legal? The only thing CA cares about is windows, blinkers, registration, and emissions. Starting with a legal car and making it emit nothing will keep them off my back.
Insurance? Quite possibly more, possibly make it be liability only, but I'm OK with that.