I would make the two parts separate. Have the person be a pawn (no need for a movement component) and make the wheelchair a vehicle.
Vehicle would contain a child actor component that would be the human pawn sitting in the chair. I would communicate between them via an interface.
The pawn would be the main controlling object as you would most likely need to sync up the hand movement with the wheel movement (if you are going for a manual rim held wheel mechanism). You would also need to apply torque cyclically and not constantly in this case (in sync with the arm movements)
For an electric wheel chair you can simplify the motions to a basic blendspace for the character for each direction. The arm could even be connected to the joystick via IK (this would allow for wheelchair model swapping with not too much complication)
The wheelchair would basically be a chaos wheeled vehicle with rear wheel drive for the most part. You would need to limit the top speed to an average wheelchair speed.
Depending on how far you want to push the simulation you would have the body react to obstacle impacts via a physics system keeping the top kinematic and the lower half simulated via a physics joint system.