If you have a dodgey back maybe the 8-10" range is a bit better in the beginning. If you go for the Saxon don't pick it up by the "handles" they are attached to the thin sides and could crease the tube, besides it would be a very awkward way to carry the tube and be a greater strain on your back.
As for moving dobs, pull it apart for anything further than a short walk out the back. Easier on you and less likely that you will have a disaster. The trolley is a good idea around where the terrain is fairly easy going but not for rough stuff. Collimation is easy when you get the hang of it so its not a worry.
I would worry about a 12" in a Corolla!! Cooldown time is longer for each successive size but the use of fans inproves this difference.
Get a Telrad for a dob.
Have fun
