@Catattack Just to reassure you, one of Motability’s strengths is how accommodating they can be when a disabled persons needs change. If you find that, over time, you physically cannot manage with the car then they will allow you to change or terminate.
It’s worth keeping in mind though that they won’t let you fit additional adaptations mid-lease, so if you think that you might need a hoist for the wheelchair and /or any other aids, it’s far easier to get them fitted up front.