find a upholstery supplier. The usually have two sizes of boards. About 11-15 bucks each. you can do each side of a car with 1 board. So door, rear inner panel and lock panel. Unless you have a large car. The stuff they have is a really heavy hard cardboard slightly over 1/8". It takes like4-5 heavy cuts with a box cutter. So you don't need a jigsaw like mdf. im currently using the stuff for mine If they have the thinner black board. That should be only for rear header panel and maybe lock panels not doors.
I used the black water prof board in my Chevelle. It didn't last long for a daily driver. I'm looking for something light to use in my race car for a backer and would really like a glue that works.
I used very thin aluminum for the rear panels in my '40 Ford Coupe. I don't know about wood or plastic, but if you do yours similar to mine, that's a pretty tight radius in the rear corners. I used a decent quality 1/8" ply for the rest of the panels. I am in So California which has fairly mild weather. We do have some heat in the summer, and I have heard of the pvc reacting to hot weather, but no experience at all with it. There is a large upholstery thread here: https://www.jalopyjournal.com/forum/threads/the-upholstery-thread-to-end-all-threads.1216758/ It's a long thread, and there is quite a bit of discussion and info on material for interior panels. Good luck on your project.
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