Instead of changing the car, how about changing the seat? Look for a maker in your area - perhaps an upholsterer, perhaps a custom car place, who can either make a new seat for your current car or alter the existing seat so that it works for you better. A really comfortable seat contoured the way that works for you will help with the bounce/vibration issues as well as giving you the right posture when seated. It may not be something they've done before, but someone who can alter car seats to help with back problems and other disabilities might start a nice business.
The other thought I've had is that you visit a breaker's yard and try the seats in lots of different cars to try to work out what works for you and what doesn't.