About the rollcage some kit has one, if not the only way it is scratch build it. There was some spare parts (I think Revell) that had a cage, but was only the central-rear.
Which kind of cage do you need? 5 points? If you don't find one I can make one for you.
About the paint, how do you paint? I mean how many hands of paint you usually do? I tell you what I do: Primer (1 hand soft), 2 hand medium, 3 hand cover all. Same with the paint, 1 soft, medium and 3 cover... And after a glass of coat, that depend of the car, and color, and result you want, you can arrive up to 10... And after I polish. And never had problem with the polish-wax.
The important is to let the color dry, in every steps.Depend of the weather, but if is warm 70-80F, in 15 minutes you can give all 3 steps. And after 2-3 hours you can cover the primer with the color... :)
If you give less hands of color, and no gloss, or the color is not dry, and you polish, could happen that if you stay too long, the color came away. Remember that all the wax are abrasive, so don't push to hard when polish.
You should make some test on a piece of plastic, like a tea spoon, just for find the way that work better for you. :)