temp paint...

If rust is a problem.......it needs to go down to bare metal in those area. If you take care of the rust areas but the paint in the other areas looks good it doesn't all have to come off. Once the car is sanded (rust fixed, old paint sanded) use the Epoxy primer (it's a sealer coat) to separate the old paint from the new. Before doing any filler

Is this car a daily driver?? Is it in a garage or shop??

Yes use the products you are gonna use in the end.....why would you want to do it twice?? Adhesion probs can occur when you mix product lines