For the 500E, from 1992 -1994 they only sold 1505 units(in the US). They were specially assembled by Porsche, had 16 inch wheels, flared fenders, W126 suspensions components for stability, and super strong brakes. That makes the 500E a very exclusive car.
I'd imagine that the value of one of these cars will appreciate fairly well over time.
Don't get rid of the 6.9! The W116 body is basically a W126 with more chrome and less rust resistance(it's even more like a W123 though).
The 6.9 is irreplaceable, and you would be kicking yourself in the butt if you got rid of it.
Besides, it would be too hard to get used to MPG again when you've been measuring the 6.9's fuel economy by GPM, and how fast the gas gauge needle shot closer to empty when you floored it, anyway.