Myy wife uses a 3-wheeler scooter called the Travel Pal. It's $530 on eBay and we like it enough we bought a second one to keep in the back of our Smart. The Travel Pal has NiMH batteries, external charger and is very well built. The handle bar folds down and the seat and seat post lift off. It fits very easily on the ledge of our Smart car and the light weight makes it easy to lift in and out.
When not using the Travel Pal scooter, she uses a 4-wheel walker with seat that folds up. It will also fit nicely on the Smart ledge when the scooter isn't there.
FWIW, anyone with a handicap that makes them stiff or less flexible, would LOVE the Smart car, since the doors open so widly and with the seat back, the opening is huge.
Nonny