[RedDevil]

I like the idea of a plugin hybrid. So much so that I want to do that too.
No doubt you've seen what Planetaire has done to his Prius.

Unlike the Prius my Insight won't be able to do prolonged 85 km/h runs in EV mode; it will restart the engine within 3 km no matter what. It will reduce fuel consumption considerably, increase boost etc. It will be more fun to drive.

Be cautious on the 18650 Lithium cell route. There is a big fire hazard if you overcharge or overheat them; they should not be combined in large packs unless very closely monitored and very well ventilated.
Several DIY PHEV hybrids have gone up in flames already.
(LiFePO4 cells are slightly heavier for the same capacity but much safer. Though even those need careful monitoring.)

OTOH they can be had very cheaply, like these: USD $ 18.99 - ICR 5000mAh 18650 Li-ion Rechargeable Battery(10pcs), Free Shipping On All Gadgets!
I'm toying with the idea to buy a bunch of those for a proof-of-concept test pack.