If I did go with plywood on the ceiling, how thin could it be without
sagging? How about OSB?

I'm leaning more away from drywall, untaped/mudded it would look very
ugly, I think?... but mudding/taping/sanding then painting that much
area sounds like more work than I want to do. I would like to get
cheap, thin plywood, paint it, then put it up with white screws. I
could live with the seams.

I will also look at other wall materials, I saw some mentioned in this
thread but I have no idea what those things are. The only building
store here is a Menards, and they don't have a lot of selection. The
new Lowes is supposed to open in November.

Someone mentioned that they just but up foam insulation board. Will
consider that too...

Thanks for all the responses.

-Ryan