If you have problems staying in the sun, my suggestion is an umbrella or parisol. You wont be the only one with one, trust me. It's a lifesaver. I have similar medical problems that keep me to the wheel chair and from doing well in heat. As for the terrain, there IOA side does have some hills, but they arent killer, and the other park is pretty flat. I didnt get the disabilities pass, but really, I didnt need it.
The only thing we had a problem with is that you couldnt do the single rider line for transformers in the wheelchair and it was about an hour shorter line to hoof it. lol.