Hi Scooby - A very late reply. Apologies for not replying earlier. I can see that you have had alot of information and hopefully you can see that driving a Z4 and other vehicles is not out of the question.
Hand controls on an automatic car is the easiest and cheapest option. Have done this on cars since 1989. There are lots of conversion people out there and the conversion can be done in a matter of hours.
Using a system to operate the clutch on a manual car is another option but more expensive. I had an electrically operated clutch on a Westfield Kit car and that was easy to use. Just a matter of pressing a button that engaged a motor which was connected to the clutch pedal via a chain. The system also sensed when slowing down and engaged the clutch to prevent stalling. :driving: Of course if you are also using hand controls for brake and throttle it takes a bit of practice getting your three hands in the right place
It is all do-able and your injuries shouldn't prevent fun! I know that I can drive just as quickly as able bodied drivers on the road. On the track is another matter as the forces involved are that much greater although with the right seat and harness's it is a real hoot!
Oh I should say that my wheelchair fits in the passenger seat when I am on my own and fits nicely in the boot when I have a passenger. I took the Z4C to Le Mans for the last two years. One year with a passenger and everything fitted including a dozen bottles of France's finest on the return.

Also taken the car from Oxford to Scotland a couple of times. Just soaks up the motorway miles.
If I can help at all please ask, more than happy. Chris