[svandamme]

sounds pretty decent for normal driving
what kind of autonomy do you think you can get out of it?

are you doing anything else , weight reductions?
do you hook the engine up to the existing tranny or direct drive? on the rear axle ( dunno how that normally works )

what kinda of costs are you looking at?

EDIT

some thing that come to mind

plug the radiator intake, smoothen things out
form a good overlap to the underside of the car ,flatten it too , and seal the bottom of the engine bay . that should reduce the drag to a minimum ( at the speed you'll be running , i don't think downforce is an issue )


solar panels ?! the 924 has a big hood, combined with the roof , i think you might be able to fit some solar panels, even if they can't fully charge the car, it gives some extra autonomy
(dunno bout the price tag though)